arm: mach-snapdragon: misc: Initialize eMMC if necessary

At the moment U-Boot produces an empty MAC address (02:00:00:00:00:00)
if the eMMC is not used by anything in U-Boot (e.g. with
CONFIG_ENV_IS_NOWHERE=y instead of having the environment on eMMC).
This happens because then there is nothing that actually initializes
the eMMC and reads the "cid" that is later accessed.

To fix this, call mmc_init() to ensure the eMMC is initialized.
There is no functional difference if the eMMC is already initialized
since then mmc_init() will just return without doing anything.

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>