riscv: Remove OF_PRIOR_STAGE from RISC-V 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.

On RISC-V some of the boards pick up the DTB from a1 and copy it in their
private gd_t.  Apart from that they copy it to prior_stage_fdt_address,  if
the Kconfig option is selected, which is unnecessary.

So let's switch the config option for those boards to OF_BOARD and define
the required board_fdt_blob_setup() for them.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>