firmware: ti_sci: Add support for Clock control

Texas Instrument's System Control Interface (TI-SCI) Message Protocol
is used in Texas Instrument's System on Chip (SoC) such as those
in keystone family K2G SoC to communicate between various compute
processors with a central system controller entity.

TI-SCI message protocol provides support for management of various
hardware entities within the SoC. Add support driver to allow
communication with system controller entity within the SoC using the
mailbox client.

In general, we expect to function at a device level of abstraction,
however, for proper operation of hardware blocks, many clocks directly
supplying the hardware block needs to be queried or configured.

Introduce support for the set of SCI message protocol support that
provide us with this capability.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>