lib: sbi: Implement SBI SSE extension

The SBI SSE extension defines a set of function that can be called to
register and handle supervisor sofwtare events. This patch implements
all of the functionality defined in the specification.

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