of/gpio: Add new method for getting gpios under different property names

This patch adds a new routine, of_get_named_gpio_flags(), which takes the
property name as a parameter rather than assuming "gpios".

of_get_gpio_flags() is modified to call of_get_named_gpio_flags() with "gpios"
as the property parameter.

Signed-off-by: John Bonesio <bones@secretlab.ca>
[grant.likely: Tidied up whitespace and tweaked kerneldoc comments.]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>