gpt: Use cache aligned buffers for gpt_h and gpt_e

gpt: Use cache aligned buffers for gpt_h and gpt_e

Before this patch one could receive following errors when executing
"gpt write" command on machine with cache enabled:

display5 factory > gpt write mmc ${mmcdev} ${partitions}
Writing GPT:
CACHE: Misaligned operation at range [4ef8f7f0, 4ef8f9f0]
CACHE: Misaligned operation at range [4ef8f9f8, 4ef939f8]
CACHE: Misaligned operation at range [4ef8f9f8, 4ef939f8]
CACHE: Misaligned operation at range [4ef8f7f0, 4ef8f9f0]
success!

To alleviate this problem - the calloc()s have been replaced with
malloc_cache_aligned() and memset().

After those changes the buffers are properly aligned (with both start
address and size) to SoC cache line.

Signed-off-by: Lukasz Majewski <lukma@denx.de>