1) In u-boot, look at /board subdirectory: there is your board-specific files should be located. There is several Marvell platforms presented, but no armada 370, so looks like you should add it by yourself. Maybe, you already have proper distribution? Examining existing code, I guess you should use GT_REG_WRITE macro (defined in /board/Marvell/common/core.h). But it can differ for armada 370.
2) In linux kernel, there is a place for board specific code too: look in /arch subdirectory. Armada 370 is based on ARM, so it should be placed into /arch/arm. There is a block of common code for all of ARM platforms, and specific subdirectories named "mach-" and "plat-". Your code should be arranged like there. In "mach-" subdirectories look for "board-" files, they are contain code executed at linux kernel start.