ARM: iop32x: request and issue reset using gpio

As the IOP GPIO driver supports gpiolib we can use the standard
GPIO calls to issue a reset of the machine instead of using the
custom gpio_line_set/config calls. Also request the GPIO when
initializing the machine.

Cc: Lennert Buytenhek <kernel@wantstofly.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Mikael Pettersson <mikpe@it.uu.se>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>