openrisc: avoid using function parameter regs in reset vector

The kernel might be invoked through the reset vector, so to
preserve parameters passed to it, temp regs that are not
in the function parameter range needs to be used.
Signed-off-by: NStefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Signed-off-by: NJonas Bonn <jonas@southpole.se>

arch/openrisc/kernel/head.S

@@ -291,9 +291,9 @@
 	/* Jump to .init code at _start which lives in the .head section
 	 * and will be discarded after boot.
 	 */
-	LOAD_SYMBOL_2_GPR(r4, _start)
-	tophys	(r3,r4)			/* MMU disabled */
-	l.jr	r3
+	LOAD_SYMBOL_2_GPR(r15, _start)
+	tophys	(r13,r15)			/* MMU disabled */
+	l.jr	r13
 	 l.nop
 
 /* ---[ 0x200: BUS exception ]------------------------------------------- */