ARM: 6377/1: supply _cansleep gpio function to U300

We have to use _cansleep gpio accessors in the MMCI driver so as
to avoid slowpath warnings, now U300 has MMCI but doesn't have
these functions in place to siply wrap the existing non-sleeping
functions into sleepable variants.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>