riscv: Add SiFive FU540 board support

This patch adds SiFive FU540 board support. For now, only
SiFive serial, SiFive PRCI, and Cadance MACB drivers are
only enabled. The SiFive FU540 defconfig by default builds
U-Boot for S-Mode because U-Boot on SiFive FU540 will run
in S-Mode as payload of BBL or OpenSBI.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 6879047..36512a8 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -14,11 +14,15 @@
 config TARGET_QEMU_VIRT
 	bool "Support QEMU Virt Board"
 
+config TARGET_SIFIVE_FU540
+	bool "Support SiFive FU540 Board"
+
 endchoice
 
 # board-specific options below
 source "board/AndesTech/ax25-ae350/Kconfig"
 source "board/emulation/qemu-riscv/Kconfig"
+source "board/sifive/fu540/Kconfig"
 
 # platform-specific options below
 source "arch/riscv/cpu/ax25/Kconfig"