cache: add ALLOC_CACHE_ALIGN_BUFFER macro

This macro is used to allocate cache line size aligned stack
buffers for use with DMA hardware.

Signed-off-by: Anton Staaf <robotboy@chromium.org>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Aneesh V <aneesh@ti.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Wolfgang Denk <wd@denx.de>