reset: add reset-simple to unify socfpga, stm32, sunxi, and zx2967

Copy reusable parts from the sunxi driver, to add a driver for simple
reset controllers with reset lines that can be controlled by toggling
bits in exclusive, contiguous register ranges using read-modify-write
cycles under a spinlock.

The following patches will replace compatible reset drivers with
reset-simple, extending it where necessary.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>