gpio: ep93xx: get rid of bogus __raw* accessors

I have no idea why this driver is using __raw* accessors for
reading and writing registers, I suspect it is just force of
habit or copy/paste. Change all to readb()/writeb() except
one chain where I used writeb_relaxed() up until the last
writeb().

Cc: Ryan Mallon <rmallon@gmail.com>
Cc: H Hartley Sweeten <hartleys@visionengravers.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>