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>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index ad6b1c0..7d1a535 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -772,6 +772,7 @@
 	  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