fdt: Correct condition for bloblist existing

On some boards, the bloblist is created in SPL once SDRAM is ready. It
cannot be accessed until that point, so is not available early in SPL.

Add a condition to avoid a hang in this case.

This fixes a hang in chromebook_coral

Fixes: 70fe2385 ("fdt: Allow the devicetree to come from a bloblist")

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Raymond Mao <raymond.mao@linaro.org>