Add board_fdt_blob_setup to return the device tree location which is
passed by prior stage in u-boot proper. The generic board_fdt_blob_setup
always returns _end, it mignt be ok because u-boot SPL would currently
put the dtb there, but it would be broken if we put the dtb to another
place and assigned the location into a1 register for u-boot proper. Use
the location passed by prior stage would make more sence, because we
actually pass the location to u-boot proper and want to use that one,
rather than the dtb which in _end.

We can't use CONFIG_OF_PRIOR_STAGE because it doens't distinguish the
implementation of u-boot SPL and u-boot proper, so u-boot SPL need to
reply on the prior stage to pass device tree location as well, but we
don't pass the DT from boot rom now. In addition, when
CONFIG_OF_PRIOR_STAGE is enabled, the u-boot-spl.bin and u-boot.itb won't
include the device tree.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>