RISC-V: KVM: add SBI extension reset callback

RISC-V: KVM: add SBI extension reset callback

Currently, only the STA extension needed a reset function but that's
going to be the case for FWFT as well. Add a reset callback that can
be implemented by SBI extensions.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20250523101932.1594077-13-cleger@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>