power domain: Introduce TI System Control Interface (TI SCI) power domain driver

Some TI Keystone 2 and K3 family of SoCs contain a system controller
(like the Power Management Micro Controller (PMMC) on 66AK2G SoCs and
the Device Management and Security Controller on AM65x 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 power domain driver that communicates to the system
controller over the TI SCI protocol for performing power management of
various devices present on the SoC. Various power domain functionalities
are achieved by the means of different TI SCI device operations provided
by the TI SCI framework.

This code is loosely based on the drivers/soc/ti/ti_sci_pm_domains.c
driver of the Linux kernel.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>