drivers: phy: Make NULL a valid phy reference

drivers: phy: Make NULL a valid phy reference

The common clock framework considers NULL a valid clock
reference. This makes handling optional clocks simple, in that if the
optional clock is not available, a NULL reference can be used in the
place of a real clock, simplifying the clock consumer.

Extend this concept to the phy consumer API. A NULL can be passed to
the release calls, the phy_init() and phy_exit() calls, and
phy_power_on() and phy_power_off() and a NOP is performed.

Signed-off-by: Andrew Lunn <[email protected]>
Tested-by: Gregory CLEMENT <[email protected]>
Acked-by: Kishon Vijay Abraham I <[email protected]>
Signed-off-by: Jason Cooper <[email protected]>