powerpc/reg: use ASM_FTR_IFSET() instead of opencoding fixup.

mftb() includes a feature fixup for CELL ppc.

Use ASM_FTR_IFSET() macro instead of opencoding the setup
of the fixup sections.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ac19713826fa55e9e7bfe3100c5a7b1712ab9526.1566999711.git.christophe.leroy@c-s.fr