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/configs/mgcoge_defconfig b/configs/mgcoge_defconfig
index c9f07d1..f4f8ae5 100644
--- a/configs/mgcoge_defconfig
+++ b/configs/mgcoge_defconfig
@@ -1,5 +1,6 @@
 CONFIG_PPC=y
 CONFIG_MPC8260=y
+CONFIG_CMD_IMMAP=y
 CONFIG_TARGET_KM82XX=y
 CONFIG_FIT=y
 CONFIG_OF_BOARD_SETUP=y