phy: mvebu-cp110-comphy: Add clocks support

There is no public clock tree that implies such dependencies between
the MG/MG-core/AXI clocks and the COMPHY IP but accessing the COMPHY
registers while one of the three clocks are disabled stalls the CPU.

This happens if, for instance, the COMPHY driver probe is deferred
(eg. the USB Vbus regulator driver is not yet visible). The MVPP2
driver which also needs these clocks (among others) will
prepare/enable the clocks, then be deferred, and disable/unprepare
them. Next COMPHY lane to be configured would produce an infinite
stall.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>