drivers/perf: RISC-V: Allow programming custom firmware events

Applications need to be able to program the SBI implementation specific
or custom firmware events in addition to the standard firmware events.
Remove a check in the driver that prohibits the programming of the custom
firmware events.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230208074314.3661406-1-mchitale@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>