Introduce initial support for Marvell Armada38x

This commit introduces initial support for Marvell Armada38x platform.
Changes:

• Add common DTS files for Armada38x SoCs and DTS file for A388-GP
• Add ARMADA38X kernel configuration
• Add option SOC_MV_ARMADA38X and set MV_PCI_PORTS
• Add list of files to compile
• Implement get_tclk(), get_sar_value(), cpu_reset() functions
• Add CPU ID and SoC numbers
• Correct ifdefs in arm/mv/timer.c

Reviewed by: ian, imp
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Michal Stanek <mst@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4210