lib: sbi_pmu: Enable noncontigous hpm event and counters

Platforms may implement hpm events/counters non contiguously but the current
implementation assumes them to be always contigous. Add a bitmap that
captures the hpm events/counters as implemented in the hardware and use
it to set the max limit of hardware counters visible to the OS. Counters
not implemented in the hardware can't be used by the OS because those
wont be described in the DT.

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