ARM: dts: k2l-netcp: add cpts refclk_mux node

KeyStone 66AK2L 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/66ak2l06


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