ep93xx: Move GPIO defines to gpio-ep93xx.h

Move the GPIO related defines out of ep93xx-regs.h and into
gpio-ep93xx.h where they belong.

Signed-off-by: Ryan Mallon <rmallon@gmail.com>
Reviewed-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: Hartley Sweeten <hsweeten@visionengravers.com>