riscv: roms: Update default bios for sifive_u machine

With the support of heterogeneous harts and PRCI model, it's now
possible to use the OpenSBI image (PLATFORM=sifive/fu540) built
for the real hardware.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

roms/Makefile

@@ -182,8 +182,8 @@ opensbi64-virt:
 opensbi64-sifive_u:
 	$(MAKE) -C opensbi \
 		CROSS_COMPILE=$(riscv64_cross_prefix) \
-		PLATFORM="qemu/sifive_u"
-	cp opensbi/build/platform/qemu/sifive_u/firmware/fw_jump.bin ../pc-bios/opensbi-riscv64-sifive_u-fw_jump.bin
+		PLATFORM="sifive/fu540"
+	cp opensbi/build/platform/sifive/fu540/firmware/fw_jump.bin ../pc-bios/opensbi-riscv64-sifive_u-fw_jump.bin
 
 clean:
 	rm -rf seabios/.config seabios/out seabios/builds