RISC-V: Probe Svinval extension form ISA string

Just like other ISA extensions, we allow callers/users to detect the
presence of Svinval extension from ISA string.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>