net: gem: Fix gem driver on 1Gbps LAN

The whole driver used 100Mbps because of zc702 rev B.
Fix problem with not setup proper clock for gem1.
This is generic approach for clk setup.

Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@ti.com>