reset: add BCM6345 reset controller driver

Add support for resetting blocks through the Linux reset controller
subsystem for BCM63xx SoCs.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <F.fainelli@gmail.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>