powerpc/powernv: Add OPAL interfaces for accessing and modifying system LED states

This patch registers the following two new OPAL interfaces calls
for the platform LED subsystem. With the help of these new OPAL calls,
the kernel will be able to get or set the state of various individual
LEDs on the system at any given location code which is passed through
the LED specific device tree nodes.

	(1) OPAL_LEDS_GET_INDICATOR     opal_leds_get_ind
	(2) OPAL_LEDS_SET_INDICATOR     opal_leds_set_ind

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Tested-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>