stmmac: intel: Enable SERDES PHY rx clk for PSE

commit 017d6250 upstream.

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>
Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>