dma-mapping: Add header file for ARCH_DMA_MINALIGN

This is defined in the asm/cache.h header file. Update this header file to
include it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Adam Ford <aford173@gmail.com> #da850-evm