riscv: change payload() to pass the config string pointer as arg0

The riscv 1.9 standard defines a textual config string to be passed
to kernels and hypervisors. Change the payload function to pass
this string in a0.

Change-Id: I3be7f1712accf2d726704e4c970f22749d3c3f36
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/17254
Tested-by: build bot (Jenkins)
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/arch/riscv/payload.S

@@ -11,10 +11,12 @@
  * GNU General Public License for more details.
  */
 
+// "return" to a payload pointed to by a1 with
+// an M-mode pointer (or, to upper levels, physical address)
+// to the config string in a0.
 	.global riscvpayload
 riscvpayload:
-	/* Jump to a0 in S-mode */
-	mv t0,a0
+	mv t0,a1
 	csrw mepc, t0
 	csrr t0, mstatus
 	li t1, ~(3<<11)