stmmac: intel: Enable SERDES PHY rx clk for PSE

EHL PSE SGMII mode requires to ungate the SERDES PHY rx clk for power up
sequence and vice versa.

Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>