docs: pmu: Update the pmu doc with removal of mcountinhbit restriction

Since commit b28f0700

, it is possible for platforms to run perf monitoring
even if mcountinhibit is not supported. Sampling still won't be possible
though as it requires sscofpmf extension.

Update the docs to remove the restriction.

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