plat/arm/common: add an additional platform power level

For platforms using multi-threaded CPUs, there can be upto four
platform power domain levels. At present, there are three platform
power domain levels that are defined for the CSS platforms. Define a
fourth level 'ARM_PWR_LVL3' as well to provide support for an
additional platform power domain level.

Change-Id: I40cc17a10f4690a560776f504364fd7277a7e72a
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>