qemuzynq: Update clock work around for QEMU 2.5+
qemuzynq: Update clock work around for QEMU 2.5+

The GEM phy in QEMU 2.5+ reports a link speed of 100Mbps, which requires
a slower clock. Change the fixed clock to provide a 25MHz reference.

Signed-off-by: Nathan Rossi <2e8aa918660411855c6d44d5bb2da677aa033255@nathanrossi.com>

nathanrossi/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx