net: dwc_eth_qos: lift parsing of max-speed DT property to common code

I have an iMX8MP with a ti,dp83867 phy in front of the eqos
interface. The phy is Gbit capable - however, the C and D differential
pairs are not physically routed to the RJ45 connector. So I need to
prevent the phy from advertising 1000Mbps.

The necessary code is almost already there in the form of a
phy_set_supported() call in eqos_start(), but the max-speed DT
property is currently only parsed in
eqos_probe_resources_stm32(). Lift that parsing to eqos_probe().

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>