lib: sbi: Add support for Supervisor Software Events extension

This extension [1] allows to deliver events from SBI to supervisor via
a software mechanism. This extension defines events (either local or
global) which are signaled by the SBI on specific signal sources (IRQ,
exceptions, etc) and are injected to be executed in supervisor mode.

[1] https://lists.riscv.org/g/tech-prs/message/798



Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>