bindings: update imx and mxs #gpio-cells

The of_get_named_gpio_flags can retrieve the second cell of
gpio-specifier as the "flags".  The imx and mxs gpio driver do not
have their own .xlate callback, which means of_gpio_simple_xlate is
used and it's a 1:1 mapping between gpio controller flags and Linux
"enum of_gpio_flags".

Update imx and mxs bindings document to make it clear.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>