boot: Assure FDT is always at 8-byte aligned address

boot: Assure FDT is always at 8-byte aligned address

The fitImage may contain FDT at 4-byte aligned address, because alignment
of DT tags is 4 bytes. However, libfdt and also Linux expects DT to be at
8-byte aligned address. Make sure that the DTs embedded in fitImages are
always used from 8-byte aligned addresses. In case the DT is decompressed,
make sure the target buffer is 8-byte aligned. In case the DT is only
loaded, make sure the target buffer is 8-byte aligned too.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>