docs/firmware: update the document

Since
commit 9c07c513

 ("firmware:Remove FW_PAYLOAD_FDT_PATH compile-time option"),
the section where FDT would be embedded in has changed from *.text* to *.rodata*,
but some places in fw_payload.md and fw.md are still *.text*.
This patch updates the document.

Signed-off-by: Yangjie Zhang <pyjmstr@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>