lantiq: VGV7510KW22BRN - set the phy clock source

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