Kconfig: Drop CONFIG_CMD_MEM

This is not actually used in U-Boot. Most likely it means
CONFIG_CMD_MEMORY so change all occurences to that.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>