dt-binding: ti: am65x-cpts: add assigned-clock and power-domains props

The CPTS clock is usually a clk-mux which allows to select CPTS reference
clock by using 'assigned-clock-parents', 'assigned-clocks' DT properties.
Also depending on integration the power-domains has to be specified to
enable CPTS IP.

Hence add 'assigned-clock-parents', 'assigned-clocks' and 'power-domains'
properties to the CPTS DT bindings to avoid dtbs_check warnings:
 cpts@310d0000: 'assigned-clock-parents', 'assigned-clocks' do not match any of the regexes: 'pinctrl-[0-9]+'
 cpts@310d0000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>