RISC-V: Support 64 bit start address

RISC-V: Support 64 bit start address

Even though the start address in ROM code is declared as a 64 bit address
for RV64, it can't be used as upper bits are set to zero in ROM code.

Update the ROM code correctly to reflect the 64bit value.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Message-Id: <20200701183949.398134-5-atish.patra@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>