The gpiomux.h header contains some SOC ifdefs.  However, the API that
is actually used by the GPIO driver only uses two functions that are
general.  Move these general definitions into a public header file.

Change-Id: Ia5df8af87dba268225598d56908e523bcfc24ef6
Signed-off-by: David Brown <davidb@codeaurora.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>