Make Marvell Armada watchdog driver more generic

Store platform dependent functions and constants in mv_wdt_config
structure and select proper configuration on runtime based on
compatible string provided in FDT.
Marvell Armada38X and ArmadaXP non-repetitive registers are moved to
generic part of code.
To support armv5 as well, use proper compatible string:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/watchdog/orion_wdt.c?h=v4.13-rc3#n456

Submitted by: Rafal Kozik <rk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D14740