ARM: tegra: acer-a500: Add aliases for MMC

MMC core now supports binding to a specific ID, which is very handy for
embedded devices, like Acer A500, because MMC ID may change depending on
kernel version or configuration which affects MMC driver probe order.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>