riscv: Use correct argument in a1 when invoking payload

Fix a bug introduced by:
820dcfceb3
riscv: Simplify payload handling

Put fdt into a1 correctly.

Change-Id: I0dea7b88fde9d9a7365cb366917747d8110b9159
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/c/31287
Reviewed-by: ron minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/riscv/payload.c

@@ -44,7 +44,7 @@ void run_payload(struct prog *prog, void *fdt, int payload_mode)
 	write_csr(mepc, doit);
 	asm volatile(
 		"mv a0, %0\n\t"
-		"mv a1, %0\n\t"
+		"mv a1, %1\n\t"
 		"mret" ::"r"(hart_id),
 		"r"(fdt)
 		: "a0", "a1");