OF: gpio: convert DT based gpio handling to new OF API ...

This creates a Linux OF API compatible counterpart of of_get_named_gpio_flags.
Existing of_get_named_gpio is converted to a static inline function, which is
in the corresponding of_gpio.h include. While at it, drivers/of/gpio.c is
also renamed to drivers/of/of_gpio.c to follow the of_ prefix naming scheme.
The new include is also added to existing users of of_get_named_gpio.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>