The size variable may not be always be a mulitple of
ARCH_DMA_MINALIGN and using it to flush cache leads to cache
misaligned warnings.

Therefore, round up the size to a multiple of ARCH_DMA_MINLAIGN
when allocating private data.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>