net: fec_mxc: Add ref clock setup support for i.MX8M Mini/Nano/Plus

The FEC ref clock frequency on i.MX8M Mini/Nano/Plus was so far configured
via ad-hoc board code. Replace that with DM clock clk_set_rate() instead.
This way, the driver claims all its required clock and sets the ref clock
rate, without any need of architecture specific register fiddling.

Signed-off-by: Marek Vasut <marex@denx.de>