riscv: clear all pending interrupts when booting

Just in case an old interrupt is pending make sure we clear everything
asserted before this kernel started.  Based on similar M-mode code in
opensbi.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nick Kossifidis <mick@ics.forth.gr>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/head.S

@@ -22,8 +22,9 @@
 
 __INIT
 ENTRY(_start)
-	/* Mask all interrupts */
+	/* Mask and clear all interrupts */
 	csrw sie, zero
+	csrw sip, zero
 
 	/* Load the global pointer */
 .option push