dts: OF_HAS_PRIOR_STAGE should depend on !BLOBLIST

When BLOBLIST is enabled, FDT is expected to be from bloblist
carried from previous stage, instead of from OF_BOARD, therefore
only enable OF_HAS_PRIOR_STAGE when BLOBLIST is disabled.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>