power: reset: Add a driver for the Microsemi Ocelot reset

The Microsemi Ocelot SoC has a register allowing to reset the MIPS core.
Unfortunately, the syscon-reboot driver can't be used directly (but almost)
as the reset control may be disabled using another register.

Cc: linux-pm@vger.kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>