firmware: Fix compile error for FW_PAYLOAD with latest GCC binutils

We get following compile error for FW_PAYLOAD with latest GCC
binutils:
fw_payload.o(.text+0x1961): 15 bytes required for alignment to 16-byte
boundary, but only 14 present

Further investigating, it turn-out to be a known issue with RISC-V
GCC binutils.
(Refer, https://github.com/riscv/riscv-gnu-toolchain/issues/298

)

As a work-around, we disable relaxation when including DTB and
PAYLOAD binary in fw_payload.S.

Reported-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Tested-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>