ARM: dts: keystone-clocks: add input fixed clocks

Add set of fixed, external input clocks definitions for TIMI0, TIMI1,
TSREFCLK clocks. Such clocks can be used as reference clocks for some HW
modules (as cpts, for example) by configuring corresponding clock muxes.
For these clocks real frequencies have to be defined in board files.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>