Convert CONFIG_CMD_IMMAP to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_IMMAP

Also move this command out of the cmd/ directory since it is
PowerPC-specific.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
index 88adb26..e09bd9a 100644
--- a/arch/powerpc/lib/Makefile
+++ b/arch/powerpc/lib/Makefile
@@ -33,6 +33,7 @@
 obj-$(CONFIG_CMD_BOOTM) += bootm.o
 obj-y	+= cache.o
 obj-y	+= extable.o
+obj-$(CONFIG_CMD_IMMAP) += immap.o
 obj-y	+= interrupts.o
 obj-$(CONFIG_CMD_KGDB) += kgdb.o
 obj-$(CONFIG_IDE) += ide.o