plat-nomadik: implement safe switch sequence for Alt-C

Setting pinmux alternative C for a GPIO pin is actually not
so easy since it ivolves setting value "1" in two registers,
and since the combined result will take effect for intermediate
values (01 or 10) this will cause glitches while you wrote one
register but have not yet written the other.

This patch implements a series of kludges including an optional
machine-specific callback to avoid glitches when changing pin
mux mode to alternative C.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>