powerpc/vdso: Merge __kernel_sync_dicache_p5() into __kernel_sync_dicache()

__kernel_sync_dicache_p5() is an alternative to
__kernel_sync_dicache() when cpu has CPU_FTR_COHERENT_ICACHE

Remove this alternative function and merge
__kernel_sync_dicache_p5() into __kernel_sync_dicache() using
standard CPU feature fixup.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4c7dcc6544882761b2b0249d7a8ec2c3a8088cb5.1601197618.git.christophe.leroy@csgroup.eu