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 <andrew@lunn.ch>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>