arm64: SMP: rework the SMP code to be enabling method agnostic

In order to introduce PSCI support, let the SMP code handle
multiple enabling methods. This also allow CPUs to be booted
using different methods (though this feels a bit weird...).

In the process, move the spin-table code to its own file.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>