x86: cache.h: Add default for CONFIG_SYS_CACHELINE_SIZE

Don't just define ARCH_DMA_MINALIGN but also CONFIG_SYS_CACHELINE_SIZE
if it's undefined. This is needed for the xhci driver to compile.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>