Documentation: dt: reset: Add TI syscon reset binding

Add TI syscon reset controller binding. This will hook to the reset
framework and use syscon/regmap to set reset bits. This allows reset
control of individual SoC subsytems and devices with memory-mapped
reset registers in a common register memory space.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>