MIPS/input: Move RB532 button to GPIO descriptors

MIPS/input: Move RB532 button to GPIO descriptors

Convert the Mikrotik RouterBoard RB532 to use GPIO descriptors
by defining a software node for the GPIO chip, then register
the button platform device with full info passing the GPIO
as a device property.

This can be used as a base to move more of the RB532 devices
over to passing GPIOs using device properties.

Use the GPIO_ACTIVE_LOW flag and drop the inversion in the
rb532_button_pressed() function.

Signed-off-by: Linus Walleij <linusw@kernel.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>