src/mainboard/emulation/qemu-power9/*: add QEMU POWER9 mainboard

Add initial implementation for booting on QEMU POWER9 emulation.

Change-Id: I079c5b9ad564024dd13296ef75c263bdc40c9d39
Signed-off-by: Yaroslav Kurlaev <yaroslav.kurlaev@3mdeb.com>
Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57079
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>

Documentation/mainboard/emulation/qemu-power9.md

@@ -0,0 +1,30 @@
+# QEMU PPC64 emulator
+This page describes how to build and run coreboot for QEMU/PPC64.
+You can configure ELF payload via `make menuconfig` (make sure "ROM chip size"
+is large enough).
+
+## Running coreboot in QEMU
+```bash
+qemu-system-ppc64 -M powernv,hb-mode=on \
+                  -cpu power9 \
+                  -bios build/coreboot.rom \
+                  -drive file=build/coreboot.rom,if=mtd \
+                  -serial stdio \
+                  -display none
+```
+
+- The default CPU in QEMU for AArch64 is a 604. You specify a suitable
+PowerPC CPU via `-cpu power9`.
+- By default Hostboot mode is off and it needs to be turned on to run coreboot
+as a firmware rather than like an OS.
+- `-bios` specifies initial program (bootloader should suffice, but whole image
+works fine too).
+- `-drive` specifies image for emulated flash device.
+
+## Building coreboot
+```bash
+make defconfig KBUILD_DEFCONFIG=configs/config.emulation_qemu_power9
+make
+```
+
+This builds coreboot with no payload.