fdt: Avoid early panic() when there is no FDT present

CONFIG_OF_CONTROL requires a valid device tree. However, we cannot call
panic() before the console is set up since the message does not appear,
and we get a silent failure.

Remove the panic from fdtdec_check_fdt() and provide a new function to
prepare the fdt for use. This will be called after the console is ready.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>