arch: armv8: fsl-layerscape: export serdes config to environment

Exports the serdes configuration as an environment variable for LS gen 3
SoCs, so it can be used in u-boot command line.  It should particularly
be useful for applying Linux DT overlays for the given serdes
configuration.
This code is called from arch_misc_init and not from the existing
serdes_init function because it depends on U-Boot environment being set
up.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>