powerpc/ps3: Add empty repository highmem routines

To avoid the need for preprocessor conditionals in C source files add a set of
empty inline repository highmem write routines to platform.h that are used when
CONFIG_PS3_REPOSITORY_WRITE is not defined.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>