net: ethernet: ti: cpts: add support for ext rftclk selection

Some CPTS instances, which can be found on KeyStone 2 1G Ethernet Switch
Subsystems, can control an external multiplexer that selects one of up to
32 clocks as time sync reference (RFTCLK) clock. This feature can be
configured through CPTS_RFTCLK_SEL register (offset: x08) in CPTS module
and can be represented as multiplexer clock.

Hence, introduce support for optional cpts-refclk-mux clock, which, once
defined will allow to select required CPTS RFTCLK by using
assigned-clock-parents DT property in board files.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>