rpi: Switch to OF_HAS_PRIOR_STAGE by default

On all Raspberry Pi platforms, we're loaded by a prior stage firmware
that has assembled and passed on a device tree binary for us to use.
Switch to using this tree by default.

Cc: Matthias Brugger <mbrugger@suse.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: François Ozog <francois.ozog@linaro.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Simon Glass <sjg@chromium.org>