davinci: omapl138_lcdk: switch to using common mmc args

Now that we have common MMC/SD boot environment
variables that can be used across TI platforms,
switch OMAP-L138 LCDK to use them.

As a nice side-effect, we get support for using
uEnv.txt on this platform.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>