Now that generate_cpu_nodes uses the cpu_compatible field to
output the compatible property, we can unify the A15 and A57
implementations, as they are strictly identical.

Move the function to fdt.c, together with most of the device
tree generation.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>