net: zynq_gem: Add SGMII dynamic config support

Add support for SGMII dynamic configuration which will takes care of
configuring SGMII in the GEM secure (GEM_CLK_CTRL) configuration
register.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/a8915186e44015959978d080a31de652f544cd4a.1648631275.git.michal.simek@xilinx.com