spl: Align FDT load address

spl: Align FDT load address

While the image size is generally a multiple of 8 bytes, this is not
actually guaranteed; some linkers (like LLD) will shave a few bytes off
of the end of output sections if there are no content bytes there. Since
libfdt imposes a hard rule of 8-byte alignment, make the SPL also be
explicit about the alignment when loading the FDT.

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