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/tuxx1_defconfig b/configs/tuxx1_defconfig
index 519472f..18b124a 100644
--- a/configs/tuxx1_defconfig
+++ b/configs/tuxx1_defconfig
@@ -1,5 +1,6 @@
 CONFIG_PPC=y
 CONFIG_MPC83xx=y
+CONFIG_CMD_IMMAP=y
 CONFIG_TARGET_TUXX1=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y