reset: add TI SYSCON based reset driver

Add a reset-controller driver for performing reset management of
various devices present on the SoC, with the reset registers shared
between devices in a common register memory space. This driver uses
the syscon/regmap frameworks to actually implement the various reset
functionalities needed by the reset consumer devices.

Signed-off-by: Andrew F. Davis <afd@ti.com>
[s-anna@ti.com: add documentation, syscon name change]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>