riscv: Allow disabling SBI extensions for Guest

We add "--disable-sbi-<xyz>" options to disable various SBI extensions
visible to the Guest. This allows users to disable deprecated/redundant
SBI extensions.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20230712163501.1769737-5-apatel@ventanamicro.com


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