Make mv_common.c generic for Marvell Armada38X and ArmadaXP

Preparation for adding Armada38X and ArmadaXP SoC to GENERIC config.
Supported platform are listed in soc_family enum.
struct decode_win_spec contains platform specific functions and constants.
Function mv_check_soc_family checks SoC type and chooses proper structure
in runtime, as well as platform-dependent functions.
Unnecessary dummy functions are removed.
Because of changing registers name to more generic new definition of
FDT_DEVMAP_MAX in mv_machdep is added.

Submitted by: Rafal Kozik <rk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D14738