ARM: davinci: omal138_lcdk: fix MMC boot breakage due to driver model conversion

ARM: davinci: omal138_lcdk: fix MMC boot breakage due to driver model conversion

commit 21af33ed0319 ("ARM: davinci: omapl138_lcdk: Enable DM_MMC")
wanted to enable DM_MMC only for U-Boot and not for SPL.

But CONFIG_DM_MMC is defined for SPL build too. Because of this
MMC device was not getting registered for SPL causing MMC/SD
boot breakage.

Instead use CONFIG_IS_ENABLED(DM_MMC) which will remain false until
CONFIG_SPL_DM_MMC is defined.

Tested-by: Adam Ford <aford173@gmail.com> #da850evm
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Tested-by: Peter Howard <phoward@gme.net.au> #omapl138_lcdk