phy: marvell: Support changing SERDES map in board file

This adds a weak definition of comphy_update_map to comphy_core,
which does nothing. If this function is defined elsewhere, for example
in board file, the board file can change some parameters of SERDES
configuration.

This is needed on Turris Mox, where the SERDES speed on lane 1 has to
be set differently when SFP module is connected and when Topaz Switch
module is connected.

This is a temporary solution. When the comphy driver for armada-3720
will be added to the kernel, the comphy driver in u-boot shall also be
updated and this should be done differently then.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: Stefan Roese <sr@denx.de>