[MMC] Support MMC version 4 cards

Version 4 of the MMC specification increased the version number of the
CID structure. None of the fields changed though so the only required
change is adding '4' to the approved list.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>