phy: core: Add documentation of phy operation order

Add documentation on phy function usage: init function must be
called before power_on; power_off must be called before exit.

Signed-off-by: Jules Maselbas <jmaselbas@kalray.eu>
Cc: Ahmad Fatoum <a.fatoum@pengutronix.de>
Cc: Amelie DELAUNAY <amelie.delaunay@foss.st.com>
Cc: Minas Harutyunyan <hminas@synopsys.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Link: https://lore.kernel.org/r/20220407102108.24211-2-jmaselbas@kalray.eu


Signed-off-by: Vinod Koul <vkoul@kernel.org>