gpio: add driver for SAMA5D2 PIOBU pins

PIOBU pins do not lose their voltage during Backup/Self-refresh.
This patch adds a simple GPIO controller for them and a
maintainer for the driver.

This driver adds support for using the pins as GPIO
offering the possibility to read/set the voltage.

Signed-off-by: Andrei Stefanescu <andrei.stefanescu@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>