net: fec_mxc: configure MDIO hold time

If the host clock frequency is higher than 100 MHz, the MDIO hold
time needs to be increased from its current setting of one cycle in
order to meet the specified minium of 10 ns.  Writing an appropriate
value to the HOLDTIME field of the MII_SPEED register achieves this.

Comment copied from Linux kernel.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Reviewed-by: Eric Nelson <eric@nelint.com>