net: phylink: add mac_select_pcs() method to phylink_mac_ops

mac_select_pcs() allows us to have an explicit point to query which
PCS the MAC wishes to use for a particular PHY interface mode, thereby
allowing us to add support to validate the link settings with the PCS.

Phylink will also use this to select the PCS to be used during a major
configuration event without the MAC driver needing to call
phylink_set_pcs().

Note that if mac_select_pcs() is present, the supported_interfaces
bitmap must be filled in; this avoids mac_select_pcs() being called
with PHY_INTERFACE_MODE_NA when we want to get support for all
interface types. Phylink will return an error in phylink_create()
unless this condition is satisfied.

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