powerpc/cpufreq: Add pr_warn() on OPAL firmware failures

Cpufreq depends on platform firmware to implement PStates.  In case of
platform firmware failure, cpufreq should not panic host kernel with
BUG_ON().  Less severe pr_warn() will suffice.

Add firmware_has_feature(FW_FEATURE_OPALv3) check to
skip probing for device-tree on non-powernv platforms.

Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Acked-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>