dt-bindings: net: renesas,etheravb: Add optional gPTP phandle for Gen4

The RAVB module on Gen4 have no gPTP clock as part of the RAVB module
itself, instead it relies on an external system wide gPTP clock. The
gPTP clock is shared with RTSN on V4H and RSWITCH on S4.

Add an optional phandle so that the RAVB driver can find and use the
gPTP clock. Ideally this should have been a mandatory property but for
backward compatible it is optional. The RAVB module is capable of
functioning without it, but can in such cases not provided PTP
functionality.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sergey Shtylyov <sergei.shtylyov@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260921175037.4084310-10-niklas.soderlund+renesas@ragnatech.se
Signed-off-by: Jakub Kicinski <kuba@kernel.org>