include: sbi: Add SBI SRST extension related defines

We extend the SBI ecall interface header for the SBI SRST extension
recently accepted in SBI specification v0.3-draft.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>