dt-bindings: doc: net: keystone-netcp: document cpts

The Keystone 2 66AK2HK/E/L 1G Ethernet Switch Subsystems contains The
Common Platform Time Sync (CPTS) module which is in general compatible with
CPTS module found on "legacy" TI AM3/4/5 SoCs. So, the basic support for
Keystone 2 CPTS is available by default, but not documented.
The Keystone 2 CPTS module supports also some additional features like time
sync reference (RFTCLK) clock selection through CPTS_RFTCLK_SEL register
(offset: x08) in CPTS module, which is modelled as multiplexer clock.

This patch adds missed binding documentation for Keystone 2 66AK2HK/E/L
CPTS module.

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>