Add docs/firmware/fw_header.md describing the layout of the 128-byte
OpenSBI firmware header, the a0/a1/a2 override flags, what a previous
booting stage is expected to put in each register, and an example of how
to patch the header. Also point at it from fw.md, next to the
description of the registers the previous booting stage passes.
Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260918081313.659655-4-zong.li@sifive.com
Signed-off-by: Anup Patel <anup@brainfault.org>
