fdt: Enable OF_HAS_PRIOR_STAGE for most boards with OF_BOARD

Use this new Kconfig instead of OF_BOARD, so we know for sure which boards
obtain their devicetree from a prior stage. Leave sandbox alone since it
does not. Also don't touch xilinx_versal_virt since it does not have a
specific TARGET Kconfig.

This option implies OF_BOARD for now, but with future work standard
passage may be used instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Add rpi_4_32b and rpi_arm64 to the list of boards converted]
Signed-off-by: Tom Rini <trini@konsulko.com>