lib: sbi_pmu: Reserve space for implementation specific firmware events

We reserve space for SBI implementation specific custom firmware
events which can be used by M-mode firmwares and HS-mode hypervisors
for their own use. This reserved space is intentionally large to
ensure that SBI implementation has enough space to accommodate
platform specific firmware events as well.

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