ARM: dts: k2hk-netcp: add cpts refclk_mux node

KeyStone 66AK2H/K 1G Ethernet Switch Subsystems, can control an external
multiplexer that selects one of up to 32 clocks for time sync reference
(RFTCLK) clock. This feature can be configured through CPTS_RFTCLK_SEL
register (offset: x08) in CPTS module and modelled as multiplexer clock.

Hence, add cpts-refclk-mux clock node which allows to mux one of SYSCLK2,
SYSCLK3, TIMI0, TIMI1, TSREFCLK clocks as CPTS reference clock [1] and
group all CPTS properties under "cpts" subnode.

[1] http://www.ti.com/lit/gpn/66ak2h14


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