power: reset: introduce oxnas-restart

Add reboot handler for Oxford OX820 chips as reboot currenly hangs on
those boards. Code is based on ox820_assert_system_reset() found in
https://github.com/kref/linux-oxnas.git

 in
arch/arm/mach-oxnas/mach-ox820.c line 181.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>