powerpc/opcodes: Add shorter macros for registers for use with PPC_RAW_xx()

Today we have __REG_Rx macros . They are mainly meant for
internal use by macros __PPC_RA() and friends macros which
allows uses like __PPC_RA(R12).

When used with PPC_RAW_xx() macros, it gives a result which is
not very readable.

Add shorter macros _Rx in order to improve readability when
used with PPC_RAW_xx() macros.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ec34d92b7c2f810622261acfeeed4b0a0f4d01bd.1621506159.git.christophe.leroy@csgroup.eu