lib: sbi: Add the bound check for events during config match

Currently, there is no sanity check for firmware event code. We don't see
any issue as Linux kernel driver does a bound check on firmware events
already. However, OpenSBI can't assume sane supervisor mode software
always. Thus, an invalid event idx can cause a buffer overflow error.
For hardware events, the match will fail for invalid event code anyways.
However, a search is unecessary if event code is invalid.

Add a event ID validation function to solve the issue.

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