Make Marvell AmadaXP timer driver more generic

Store pointers to SoC specific functions in mv_timer_config structure
and determine proper config in runtime based on compatible string from FDT.
Compatible string for ArmadaXP timers is changed to match Linux FDT.
Armada 38x uses generic Cortex-A9 timer and separate watchdog drivers, so
it does not need to be supported by timer driver.

Submitted by: Rafal Kozik <rk@semihalf.com>
Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D14741