power: reset: Add AT91 reset driver

Implement the reset behaviour of the various AT91 SoCS in drivers/power/reset.

It used to be (and still is) located in arch/arm/mach-at91, and in order to
preserve bisectability is not removed yet, but every board should be converted
to use this driver instead.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>