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