plat/arm/sgi: allow value of PLAT_MAX_PWR_LVL to be platform specific

For platforms with multi-threaded CPUs, the number of power domains
supported would be more than the value currently defined by
PLAT_MAX_PWR_LVL. So move the PLAT_MAX_PWR_LVL macro to platform
specific code and let the platform define the number of power domain
levels.

Change-Id: I21c0682e62b397860b2999031a0c9c5ce0d28eed
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>