dt-bindings: arm: alpine: Move CPU control related binding to cpu-enable-method/al,alpine-smp

It is best practice to have 1 binding per file, so board level bindings
should be separate for various misc SoC bindings. Move the Alpine CPU
control to al,alpine-smp and we can also remove a cross reference.

Cc: Tsahee Zidenberg <tsahee@annapurnalabs.com>
Cc: Antoine Tenart <antoine.tenart@free-electrons.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>