plat/css: allow platforms to define the system power domain level

The CSS_SYSTEM_PWR_DMN_LVL macro that defines the system power domain
level is fixed at ARM_PWR_LVL2 for all CSS platforms. However, the
system power domain level can be different for CSS platforms that
use multi-threaded CPUs.

So, in preparation towards adding support for platforms that use
multi-threaded CPUs, refactor the definition of CSS_SYSTEM_PWR_DMN_LVL
such that CSS_SYSTEM_PWR_DMN_LVL is uniquely defined for each of the
CSS platform.

Change-Id: Ia837b13f6865e71da01780993c048b45b7f36d85
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>