riscv: fix scratch register clearing in M-mode.

This patch fixes that the sscratch register clearing in M-mode. It cleared
sscratch register in M-mode, but it should clear mscratch register. That will
cause kernel trap if the CPU core doesn't support S-mode when trying to access
sscratch.

Fixes: 9e806356 ("riscv: clear the instruction cache and all registers when booting")
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/kernel/head.S

@@ -246,7 +246,7 @@ ENTRY(reset_regs)
 	li	t4, 0
 	li	t5, 0
 	li	t6, 0
-	csrw	sscratch, 0
+	csrw	CSR_SCRATCH, 0
 
 #ifdef CONFIG_FPU
 	csrr	t0, CSR_MISA