reset: Add the TI SCI reset driver

Some TI Keystone family of SoCs contain a system controller (like the
Power Management Micro Controller (PMMC) on 66AK2G SoCs) that manage
the low-level device control (like clocks, resets etc) for the various
hardware modules present on the SoC. These device control operations
are provided to the host processor OS through a communication protocol
called the TI System Control Interface (TI SCI) protocol.

This patch adds a reset driver that communicates to the system
controller over the TI SCI protocol for performing reset management
of various devices present on the SoC. Various reset functionalities
are achieved by the means of different TI SCI device operations
provided by the TI SCI framework.

Signed-off-by: Andrew F. Davis <afd@ti.com>
[s-anna@ti.com: documentation changes, revised commit message]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
[p.zabel@pengutronix.de: const struct reset_control_ops]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>