reset: Add Sunplus SP7021 reset driver

Add reset driver for Sunplus SP7021 SoC.

Signed-off-by: Qin Jian <qinjian@cqplus1.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>