ARM/orion/gpio: Make use of for_each_requested_gpio()

Make use of for_each_requested_gpio() instead of home grown analogue.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Gregory Clement <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20200615150545.87964-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>