powerpc/reg.h: delete duplicated word

Drop the repeated word "a".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200726003809.20454-8-rdunlap@infradead.org