include: Use _UL() and _ULL() for defines in riscv_encoding.h

The riscv_encoding.h is shared with assembly sources so we use
_UL() and _ULL() for register fields related defines.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>