spl: riscv: opensbi: Error on misaligned FDT

spl: riscv: opensbi: Error on misaligned FDT

libfdt 1.6.1+ requires the FDT to be 8-byte aligned and returns an error
if not. OpenSBI 1.0+ includes this version of libfdt and will also
reject misaligned FDTs.

However, OpenSBI cannot indicate the error to the user: since it cannot
access the serial console, it can only silently hang. This proved very
difficult to diagnose without proper debugging facilities. Therefore,
give the U-Boot SPL, which *can* print error messages, an additional
check for proper FDT alignment. Hopefully this saves a lot of
development cycles if another developer encounters alignment problems.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>