MMC: add weak function to detect MMC/SD card

Most controllers can check if there is a card in the slot.
However, they require pins that could be not available because
required by other functions and the detection of a card must
be performed in another way. This patch adds a weak function
that a board can implement to add its internal custom way
to check the presence of a MMC/SD card.

Signed-off-by: Stefano Babic <sbabic@denx.de>