ldrp-v2: Add new method of finding number of CPUs as of kernel 4.5

As of commit c4c54dd1 (kernel/cpu.c: change type of cpu_possible_bits
and friends), cpu_present_bits no longer exists. Add new method of finding
number of cpus and also change other functions to use the get_num_cpus
API instead of defining it again.

Change-Id: I6d38d505ba7674b94e8481e6679226803eba977e