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/Kconfig b/arch/powerpc/Kconfig
index 3d2c131..eae94c8 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -49,6 +49,8 @@
 
 endchoice
 
+source "arch/powerpc/lib/Kconfig"
+
 source "arch/powerpc/cpu/mpc512x/Kconfig"
 source "arch/powerpc/cpu/mpc5xx/Kconfig"
 source "arch/powerpc/cpu/mpc5xxx/Kconfig"