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: 9e80635619b5 ("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>