ARM: dts: k2e-clocks: add input ext. fixed clocks tsipclka/b

Add set of fixed, external input clocks definitions for TSIPCLKA, TSIPCLKB
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>