riscv: Add X register names to gpr-nums

When encoding instructions it's sometimes necessary to set a
register field to a precise number. This is easiest to do using
the x<num> naming.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>

riscv: add gpr-num.h

This defines the mapping from ABI names to X registers.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>