arm64: topology: Define arch_get_cpu_efficiency() API for scheduler

On a HMP system, scheduler needs to know efficiency factor
(instructions-per-cycle) for various cpus. This is so that scheduler
can estimate bandwidth consumption of tasks on each cpu, based on
their efficiency factor.

This patch defines arch_get_cpu_efficiency() API in ARM64
architecture.  It depends on hard-coded "efficiency" factor for
various cpu types (available in 'table_efficiency' data structure) and
device-tree providing information on cpu-type for every cpu.

Change-Id: Ied43ead650ab85b63c232bec14dde500cbcc0f7a
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>