firmware: Add common FW_FDT_PATH compile-time option

Currently, only FW_PAYLOAD has mechanism to embed external
FDT using FW_PAYLOAD_FDT_PATH compile-time option.

This patch adds a common FW_FDT_PATH compile-time option to
embed external FDT for all OpenSBI firmwares (i.e FW_JUMP,
FW_PAYLOAD, and FW_DYNAMIC).

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>