ARM: sched: Use device-tree to provide fast/slow CPU list for HMP

ARM: sched: Use device-tree to provide fast/slow CPU list for HMP

We can't rely on Kconfig options to set the fast and slow CPU lists for
HMP scheduling if we want a single kernel binary to support multiple
devices with different CPU topology. E.g. TC2 (ARM's Test-Chip-2
big.LITTLE system), Fast Models, or even non big.LITTLE devices.

This patch adds the function arch_get_fast_and_slow_cpus() to generate
the lists at run-time by parsing the CPU nodes in device-tree; it
assumes slow cores are A7s and everything else is fast. The function
still supports the old Kconfig options as this is useful for testing the
HMP scheduler on devices without big.LITTLE.

This patch is reuse of a patch by Jon Medhurst <tixy@linaro.org> with a
few bits left out.

Signed-off-by: Morten Rasmussen <Morten.Rasmussen@arm.com>