powerpc/64: flush_inval_dcache_range() becomes flush_dcache_range()

On most arches having function flush_dcache_range(), including PPC32,
this function does a writeback and invalidation of the cache bloc.

On PPC64, flush_dcache_range() only does a writeback while
flush_inval_dcache_range() does the invalidation in addition.

In addition it looks like within arch/powerpc/, there are no PPC64
platforms using flush_dcache_range()

This patch drops the existing 64 bits version of flush_dcache_range()
and renames flush_inval_dcache_range() into flush_dcache_range().

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>