powerpc/cpuidle: Fix parsing of idle state flags from device-tree

Flags from device-tree need to be parsed with accessors for
interpreting correct value in little-endian.

Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Reviewed-by: Preeti U. Murthy <preeti@linux.vnet.ibm.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>