lib: sbi: Enable mie.MEIE bit for IPIs based on external interrupts.

We can have IPIs based on external interrupts provided by devices
such as AIA IMSIC so we should enable mie.MEIE bit at appropriate
places in generic library.

Signed-off-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>