dt: describe base reset signal binding

This binding is intended to represent the hardware reset signals present
internally in most IC (SoC, FPGA, ...) designs.
It consists of a binding for a reset controller device (provider), and a
pair of properties, "resets" and "reset-names", to link a device node
(consumer) to its reset controller via phandle, similarly to the clock
and interrupt bindings.

The reset controller has all information necessary to reset the consumer
device. That could be provided via device tree, or it could be implemented
in hardware.
The aim is to enable device drivers to request a framework API to issue a
reset simply by providing their struct device pointer as the most common
case.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Rob Herring <rob.herring@calxeda.com>