riscv: Add Zbc extension support

When the Zbc extension is available expose it to the guest
via device tree so that guest can use it.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20240325153141.6816-4-apatel@ventanamicro.com


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