riscv: Enable CONFIG_OF_BOARD_FIXUP by default for OF_SEPARATE

Starting from OpenSBI v0.7, the SBI firmware inserts/fixes up the
reserved memory node for PMP protected memory regions. All RISC-V
boards need to copy the reserved memory node from the device tree
provided by the firmware to the device tree used by U-Boot.

Turn on CONFIG_OF_BOARD_FIXUP by default for OF_SEPARATE.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d9854f5..ff8a9f8 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -273,4 +273,7 @@
 	int
 	default 14
 
+config OF_BOARD_FIXUP
+	default y if OF_SEPARATE
+
 endmenu