spl: riscv: opensbi: fix check of PAYLOAD_ARGS_ADDR

When Falcon Mode is enabled on RISC-V, use CONFIG_VAL
to check PAYLOAD_ARGS_ADDR, not CONFIG_IS_ENABLED.

Fixes: 10c4ab89 ("spl: riscv: falcon: move fdt blob to specified address")
Signed-off-by: Randolph <randolph@andestech.com>
Tested-by: Leo Yu-Chi Liang <ycliang@andestech.com>