net: phy: Add helper to derive the number of ports from a phy mode

Some phy modes such as QSGMII multiplex several MAC<->PHY links on one
single physical interface. QSGMII used to be the only one supported, but
other modes such as QUSGMII also carry multiple links.

This helper allows getting the number of links that are multiplexed
on a given interface.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>