platform: qemu: Set FW_JUMP_ADDR and FW_PAYLOAD_OFFSET as-per XLEN

The current 4MB aligned FW_JUMP_ADDR and FW_PAYLOAD_OFFSET breaks
U-Boot on QEMU virt and sifive_u machines.

Instead of using 4MB aligned for both 32bit and 64bit systems, we
use different values based compiler XLEN. Another advantage of this
approach will be that our fw_payload.bin will smaller for 64bit
systems.

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