drivers/net/phy: introduce phy_set_supported()

drivers/net/phy: introduce phy_set_supported()

This new function will allow MAC drivers to override supported
capabilities of the phy. It is required when MAC cannot handle all
speeds supported by phy.

For example phy supports up-to 1Gb connections while MAC may only work
in modes up to 100 or even 10 Mbit/sec.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>