blob: payload: Add FW_PAYLOAD_FDT_PATH option

This patch extends fw_payload blob to provide FW_PAYLOAD_FDT_PATH
option using which we can embed custom FDT in .text section of
fw_payload blob. In other words, FW_PAYLOAD_FDT_PATH is an option
to forcefully override FDT passed by previous booting stage in
a1 register.

Signed-off-by: Anup Patel <anup.patel@wdc.com>