firmware: Remove FW_PAYLOAD_FDT_PATH compile-time option

The FW_PAYLOAD_FDT_PATH compile-time option is replaced by
FW_FDT_PATH compile-time option which is more flexible and
common across all OpenSBI firmwares.

This patch removes FW_PAYLOAD_FDT_PATH and updates related
documentation to use FW_FDT_PATH.

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