common.h: Introduce DEFINE_CACHE_ALIGN_BUFFER

This is the out-of-function-scope counterpart of
ALLOC_CACHE_ALIGN_BUFFER.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>
[ilya.yanok]: added missing <linux/compiler.h> include and
{DEFINE,ALLOC}_ALIGN_BUFFER macros allowing explicit alignment
specification.
Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>