mvebu: cp110: introduce COMPHY porting layer ...

Some of COMPHY parameters depends on the hw connection between the SoC
and the PHY, which can vary on different boards e.g. due to different
wires length. Define the "porting layer" with some defaults
parameters. It ease updating static values which needs to be updated due
to board differences, which are now grouped in one place.

Example porting layer for a8k-db is under:
plat/marvell/a8k/a80x0/board/phy-porting-layer.h

If for some boards parameters are not defined (missing
phy-porting-layer.h), the default values are used
(drivers/marvell/comphy/phy-default-porting-layer.h)
and the following compilation warning is show:
"Using default comphy params - you may need to suit them to your board".

The common COMPHY driver code is extracted in order to be shared with
future COMPHY driver for A3700 SoC platforms

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Signed-off-by: Igal Liberman <igall@marvell.com>
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>

docs: Add Marvell build and porting documents ...

Change-Id: I341440701b7e5e3555e604dd9d0a356795e6c4fb
Signed-off-by: Hanna Hawa <hannah@marvell.com>
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>