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>
diff --git a/configs/bcm7445_defconfig b/configs/bcm7445_defconfig
index d5dd4b7..3726abd 100644
--- a/configs/bcm7445_defconfig
+++ b/configs/bcm7445_defconfig
@@ -26,7 +26,7 @@
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_EXT4=y
 CONFIG_CMD_FS_GENERIC=y
-CONFIG_OF_PRIOR_STAGE=y
+CONFIG_OF_BOARD=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y