clk: keystone: Add sci-clk driver support

In K2G, the clock handling is done through firmware executing on a
separate core. Linux kernel needs to communicate to the firmware
through TI system control interface to access any power management
related resources, including clocks.

The keystone sci-clk driver does this, by communicating to the
firmware through the TI SCI driver. The driver adds support for
registering clocks through DT, and basic required clock operations
like prepare/get_rate, etc.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
[sboyd@codeaurora.org: Make ti_sci_init_clocks() static]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>