riscv: Add Ssaia extension support

When the Ssaia extension is available expose it to the guest.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20230712163501.1769737-9-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>