dm: blk: mmc: Tidy up some Makefile rules for SPL

dm: blk: mmc: Tidy up some Makefile rules for SPL

Use the correct SPL_TPL_ variable so that these features can be enabled in
TPL and VPL as needed.

Disable it by default in TPL to avoid any code-size increase. No boards
are actually using it since the Makefile rules don't allow including
drivers/block/ with TPL_DM enabled. It can be manually enabled as needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>