kexec: align the extended size of dtb

When loading dtb on arm platform, we may need change strings in dtb
in function setup_dtb_prop. If length of new string is larger than that
of the old one. We should extend the dtb by "FDT_TAGALIGN"ed size.
If not, kexec may fail to load the capture kernel.

Signed-off-by: Liu Hua <sdu.liu@huawei.com>
Signed-off-by: Simon Horman <horms@verge.net.au>