sched: walt/energy: use topology_get_cpu_scale to get the efficiency

topology_get_cpu_scale can be used to get the efficiency instead of
relying on arch_get_cpu_efficiency.

Change-Id: I6b68f1bfb58edf7f6d6ab0f1a16fa226c6367eab
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>