lib: sbi: Detect and print MHPM counters at boot-time

A RISC-V platform can leave unimplemented MHPM counters hard-wired
to zero. We extend hart_detect_features() to detect MHPM counters
which are accessible and not hard-wired to zero. We also print
number of available MHPM counters as part of boot prints.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>