powerpc/time: Remove ifdef in get_dec() and set_dec()

Move SPRN_PIT definition in reg.h.

This allows to remove ifdef in get_dec() and set_dec() and
makes them more readable.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3c9a6eb0fc040868ac59be66f338d08fd017668d.1601549945.git.christophe.leroy@csgroup.eu