powerpc: Make LOWMEM_CAM_NUM depend on FSL_BOOKE

The recent addition of CONFIG_LOWMEM_CAM_BOOL and
CONFIG_LOWMEM_CAM_NUM cause the latter to show up in configs
that do not need it during 'make oldconfig'.  Make LOWMEM_CAM_NUM
depend on FSL_BOOKE.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>