RISC-V: Detect Smstateen extension

Extend the ISA string parsing to detect the Smstateen extension. If the
extension is enabled then access to certain 'state' such as AIA CSRs in
VS mode is controlled by *stateen0 registers.

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