Function 0 is shared between all drivers using a particular card.
Currently drivers read and write to function 0 directly, but eventually
function 0 will only be accessible through one of the methods added
here.

Bug: ZX-3879
Test: runtests -t sdmmc-test
Change-Id: If1dfff0eba05a4b1d6ff255b4800ab4843b7ea78