arch/arm: re-order cores choice

arch/arm: re-order cores choice

Currently, the logic for ordering the ARM cores in the choice is all
but obvious. ;-)

Reorder the choice by architecture generation, starting with armv4,
ending with armv8.

Add a comment before each generation, just for ease of use. Add a
separate comment for armv7a and armv7m.

Finally, order cores alphabetically inside the same generation (except
for armv7m cores, listed after all armv7a cores).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>