riscv: Setup reserved-memory node for FU540

FU540 uses OF_SEPARATE instead of OF_PRIOR_STAGE.

Enable OF_BOARD_FIXUP to update the DT with reserved-memory node.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/sifive_fu540_defconfig b/configs/sifive_fu540_defconfig
index 6d61e6c..f805aac 100644
--- a/configs/sifive_fu540_defconfig
+++ b/configs/sifive_fu540_defconfig
@@ -9,6 +9,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_DISPLAY_BOARDINFO=y
+CONFIG_OF_BOARD_FIXUP=y
 CONFIG_DEFAULT_DEVICE_TREE="hifive-unleashed-a00"
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DM_MTD=y