riscv: Allow disabling SBI STA extension for Guest

We add "--disable-sbi-sta" options to allow users disable SBI steal-time
extension for the Guest.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20240325153141.6816-11-apatel@ventanamicro.com


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