net: phylink: add generic validate implementation

Add a generic validate() implementation using the supported_interfaces
and a bitmask of MAC pause/speed/duplex capabilities. This allows us
to entirely eliminate many driver private validate() implementations.

We expose the underlying phylink_get_linkmodes() function so that
drivers which have special needs can still benefit from conversion.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>