arch/arm: Enable FIT payloads

Implements fit_payload_arch for the arm (aarch32) architecture, so that
FIT images can be used. The implementation is very similar to the
existing implementations for arm64 and riscv, and has mostly been
lifted from these other ports.

TEST: Booted Beaglebone Black (in progress port, to be submitted soon!)
with a FIT image containing a 5.4 kernel, dtb and initramfs.

Change-Id: I6b50c6f06b83c00a5b3622b5bbafe67130b6d233
Signed-off-by: Sam Lewis <sam.vr.lewis@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44377
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

Documentation/lib/payloads/fit.md

@@ -5,6 +5,7 @@
 
 ## Supported architectures
 
+* aarch32
 * aarch64
 * riscv
 
@@ -26,6 +27,13 @@ The section must be named in order to be found by the FIT parser:
 
 The FIT parser needs architecure support.
 
+### aarch32
+The source code can be found in `src/arch/arm/fit_payload.c`.
+
+On aarch32 the kernel (a section named 'kernel') must be in **Image**
+format and it needs a devicetree (a section named 'fdt') to boot.
+The kernel will be placed close to "*DRAMSTART*".
+
 ### aarch64
 The source code can be found in `src/arch/arm64/fit_payload.c`.