[POWERPC] Remove unneeded page_is_ram export

arch/powerpc/mm/mem.c exports page_is_ram, which is not used anywhere
that could be modular.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>