VGV7510KW2 with VRX288 v1.2 has brnboot 1.8 installed. Starting with
this brnboot version, the "GPHY Clock Source" isn't set anymore by
brnboot, with the result that xrx200-net fails to probe/initialize the
phys.

Use the phy clock source device tree binding to specify the clock source.

Signed-off-by: Mathias Kresin <openwrt@kresin.me>

SVN-Revision: 49284