powerpc: Don't include asm/ppc_asm.h in other headers

asm/ppc_asm.h is not needed in any of the header it is included.

It is only needed by irq.c. Include it there and remove it from
other headers.

word-at-a-time.h only need ex_table.h, so include it instead.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e2d7b96547037f852c7ed164e4f79e8918c2607a.1651828453.git.christophe.leroy@csgroup.eu