phy: core: Allow phy_pm_runtime_xxx API calls with NULL phy

phy_init() and phy_exit() calls, and phy_power_on() and
phy_power_off() already accept NULL as valid PHY reference
and act as NOP. Extend same concept to phy runtime_pm APIs
to keep drivers (e.g. dwc3) code simple while dealing with
optional PHYs.

Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>