ARM: bL_switcher: remove assumptions between logical and physical CPUs

Up to now, the logical CPU was somehow tied to the physical CPU number
within a cluster.  This causes problems when forcing the boot CPU to be
different from the first enumerated CPU in the device tree creating a
discrepancy between logical and physical CPU numbers.

Let's make the pairing completely independent from physical CPU numbers.

Let's keep only those logical CPUs with same initial CPU cluster to create
a uniform scheduler profile without having to modify any of the probed
topology and compute capacity data.  This has the potential to create
a non contiguous CPU numbering space when the switcher is active with
potential impact on buggy user space tools.  It is however better to fix
those tools rather than making the switcher code more intrusive.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>