lib: sbi: Map only the counters enabled in hardware

The counter mapping in DT may be incorrect if all the counters specified
in the mapping are actually not physically present in the hardware.
OpenSBI should only keep a mapping of counters enabled in hardware and
defined in DT. This assume that all the programmable hpmcounters are
consecutive as it doesn't make sense to build a system with sparse
hpmcounters.

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