net: xilinx: axiethernet: Enable dynamic MDIO MDC

MDIO spec does not require an MDC at all times, only when MDIO
transactions are occurring. This patch allows the xilinx_axienet
driver to disable the MDC when not in use, and re-enable it when
needed. It also simplifies the driver by removing MDC disable
and enable in device reset sequence.

Signed-off-by: Clayton Rayment <clayton.rayment@xilinx.com>
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>