net: dwc_eth_qos: Enable clock in probe

Enable DWC IP clock in driver probe, so the MII access is possible even
outside of active network transfers. This is particularly useful when
using 'mii' or 'mdio' commands to explore PHY state, neither of which
works with DWMAC currently due to the disabled clock.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>