Documentation: bindings: Add timestamp-valid-window item to Neutrino

Add timestamp-valid-window property to the device tree for Neutrino.
This is used to set the EAVB Timestamp Control register to control
which TX packets are held by the Neutrino until the timestamp.

CRs-Fixed: 1056978
Change-Id: Id038a66059be1c092298861e8de61386871bef80
Signed-off-by: Samuel Ahn <sahn@codeaurora.org>