blob: payload: Introduce FW_PAYLOAD_OFFSET compile time option

This patch remove hard-coding of payload location in
fw_payload.elf.ldS by adding compile-time option
FW_PAYLOAD_OFFSET.

With the new compile-time option, payload will be placed
at PLAT_TEXT_START + FW_PAYLOAD_OFFSET address.

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