net: fec_mxc: add support for i.MX8X

Add compatible property and enable the FEC ipg clock when probing
on i.MX8X. Add specific function for reading FEC clock rate via
clock driver when configuring MII speed register. Allow FEC_MXC
selection for i.MX8.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>