reset: Add a driver for the reset controller on the AR71XX/AR9XXX

The AR71XX/AR9XXX SoC have a simple reset controller with one bit per
reset line.

Signed-off-by: Alban Bedel <albeu@free.fr>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>