board: arm: Remove OF_PRIOR_STAGE from the remaining Arm boards

At some point back in 2018 prior_stage_fdt_address and OF_PRIOR_STAGE got
introduced,  in order to support a DTB handed over by an earlier stage boo
loader.  However we have another option in the Kconfig (OF_BOARD) which has
identical semantics.

So let's remove the option in an effort to simplify U-Boot's config and DTB
management,  and use OF_BOARD instead.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>