gpio: omap: Add an omap prefix to all functions

The GPIO OMAP driver didn't have a consistent naming scheme for
all its functions. Some of them had an omap prefix while others
didn't. There are many advantages on having a separate namespace
for driver functions so let's add an "omap" prefix to all of them.

Signed-off-by: Javier Martinez Canillas <jmartinez@softcrates.net>
Acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>