regmap-mmio is used in Linux for clocked memory mapped I/O regions.
Port it over, so we can more easily port drivers using it.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

This adds initial regmap support. Function prototypes are from
the Kernel, the implemention is mostly rewritten. Currently the
regmap support is limited to the bare minimum to get started.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>