riscv: Add zbb extension support

The zbb extension allows software to use basic bitmanip instructions.
Let us add the zbb extension to the Guest device tree whenever it is
supported by the host.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20230712163501.1769737-7-apatel@ventanamicro.com


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