phylib: Add phy_set_max_speed helper

Add a helper to allow ethernet drivers to limit the speed of a phy
(that they are attached to).

This mainly involves factoring out the business-end of
of_set_phy_supported() and exporting a new symbol.

This code seems to be open coded in several places, in several different
variants.

It is is envisaged that this will be used in situations where setting the
"max-speed" property in DT is not appropriate, e.g. because the maximum
speed is not a property of the phy hardware.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>