mtd: nand: mxs: fix cache alignment for cache lines >32

Currently the command buffer gets allocated with a size of 32 bytes.
This causes warning messages on systems with cache lines bigger than
32 bytes:
CACHE: Misaligned operation at range [9df17a00, 9df17a20]

Define command buffer to be at least 32 bytes, but more if cache
line is bigger.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>