riscv: sifive: Rename fu540 board to unleashed

In preparation to add SiFive Unmatched board support, let's rename
the existing fu540 board to unleashed.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/arch/riscv/dts/Makefile b/arch/riscv/dts/Makefile
index 01331b0..8138d89 100644
--- a/arch/riscv/dts/Makefile
+++ b/arch/riscv/dts/Makefile
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0+
 
 dtb-$(CONFIG_TARGET_AX25_AE350) += ae350_32.dtb ae350_64.dtb
-dtb-$(CONFIG_TARGET_SIFIVE_FU540) += hifive-unleashed-a00.dtb
+dtb-$(CONFIG_TARGET_SIFIVE_UNLEASHED) += hifive-unleashed-a00.dtb
 dtb-$(CONFIG_TARGET_SIPEED_MAIX) += k210-maix-bit.dtb
 dtb-$(CONFIG_TARGET_MICROCHIP_ICICLE) += microchip-mpfs-icicle-kit.dtb