This value is about DMA zone size of Linux DMA zone size, this value can set in CONFIG_DMA_ZONE_SIZE, This value is set by kernel .config, vi your .config file to see how much, how kernel use this value?
The arch memory.h header file will use your define function named arch_adjust_zones() to adjust global value zone_size[] to adjust you custom memory zone infomation.
files:
arch/arm/include/asm/memory.h : arch_adjust_zones
arch/arm/mach-xxx/include/mach/memory.h arch_adjust_zones