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

Since commit b28f070, 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>

docs/pmu_support.md

@@ -6,9 +6,6 @@ capability of performance analysis tools such as perf if SBI PMU extension is
 enabled. The OpenSBI implementation makes the following assumptions about the
 hardware platform.
 
- * MCOUNTINHIBIT CSR must be implemented in the hardware. Otherwise, SBI PMU
-extension will not be enabled.
-
  * The platform must provide information about PMU event to counter mapping
 via device tree or platform specific hooks. Otherwise, SBI PMU extension will
 not be enabled.