mainboard/emulation/qemu-sbsa: Add qemu-sbsa board

Add coreboot support for qemu's sbsa-ref (Server Base System
Architecture) machine (-m sbsa-ref).

The qemu-sbsa coreboot port runs on EL2 and is the payload of the
EL3 firmware (Arm Trusted Firmware).

Note that, coreboot expects a pointer to the FDT in x0. Make sure
to configure TF-A to handoff the FDT pointer.

Example qemu commandline:

  qemu-system-aarch64 -nographic -m 2048 -M sbsa-ref \
                      -pflash <path/to/TFA.fd> \
                      -pflash <path/to/coreboot.rom>

The Documentation can be found here:
Documentation/mainboard/emulation/qemu-sbsa.md

Change-Id: Iacc9aaf065e0d153336cbef9a9b5b46a9eb24a53
Signed-off-by: David Milosevic <David.Milosevic@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79086
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>

Documentation/mainboard/emulation/qemu-sbsa.md

@@ -0,0 +1,42 @@
+# QEMU SBSA emulator
+This page describes how to build and run ```coreboot``` for QEMU's sbsa-ref machine.
+The qemu-sbsa ```coreboot``` image acts as BL-3.3 for Arm Trusted Firmware (```TF-A```) and
+mainly takes care of setting up SMBIOS and ACPI tables, hence, in order to boot,
+you also need to supply a ```TF-A``` image.
+
+## Building TF-A
+
+You can build ```TF-A``` from source by fetching
+```
+https://github.com/ARM-software/arm-trusted-firmware
+```
+and building the qemu-sbsa platform
+```
+PLAT=qemu_sbsa
+```
+Upon entry, ```coreboot``` expects a FDT pointer in x0, so make sure to compile ```TF-A``` with
+```
+ARM_LINUX_KERNEL_AS_BL33=1
+```
+This will force ```TF-A``` to pass a pointer to the FDT in x0.
+
+## Building coreboot
+
+Simply select the qemu-sbsa board and, optionally, configure a payload. We recommend
+the ```leanefi``` payload. ```leanefi``` will setup a minimal set of UEFI services, just enough
+to boot into a linux kernel.
+
+## Running coreboot in QEMU
+
+Once you have obtained ```TF-A``` and ```coreboot``` images, launch qemu via
+
+```bash
+qemu-system-aarch64 -nographic -m 1024 -M sbsa-ref -pflash <path/to/TFA.fd> \
+                                                   -pflash <path/to/coreboot.rom>
+```
+
+## LBBR bootflow
+
+arm and 9elements worked together in order to create a LBBR compliant bootflow
+consisting of ```TF-A```, ```coreboot```, ```leanefi``` and ```LinuxBoot```. A proof of concept
+can be found here https://gitlab.arm.com/systemready/firmware-build/linuxboot/lbbr-coreboot-poc

Documentation/mainboard/index.md

@@ -90,6 +90,7 @@ The boards in this section are not real mainboards, but emulators.
 Spike RISC-V emulator <emulation/spike-riscv.md>
 QEMU RISC-V emulator <emulation/qemu-riscv.md>
 QEMU AArch64 emulator <emulation/qemu-aarch64.md>
+QEMU SBSA emulator <emulation/qemu-sbsa.md>
 QEMU x86 Q35 <emulation/qemu-q35.md>
 QEMU x86 PC <emulation/qemu-i440fx.md>
 QEMU POWER9 <emulation/qemu-power9.md>