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

arch/powerpc/Kconfig

@@ -772,6 +772,7 @@ config LOWMEM_CAM_NUM_BOOL
 	  Say N here unless you know what you are doing.
 
 config LOWMEM_CAM_NUM
+	depends on FSL_BOOKE
 	int "Number of CAMs to use to map low memory" if LOWMEM_CAM_NUM_BOOL
 	default 3