Fixup Broadcom CPU enable method

This is preparation for supporting the quad-core BCM23550 chip.

Documentation/devicetree/bindings/arm/cpus.txt states that "enable-method" should be a property of the "cpu" node rather than the "cpus" node.

Commit 84320e1a635fcf90cff4185f029ce9e31bf1d4a7
("ARM: BCM: Clean up SMP support for Broadcom Kona") moved the "secondary-boot-reg" property from the "cpus" node to the individual "cpu" nodes but negelected to actually support multiple "secondary-boot-reg" properties.

This patchset moves the enable-method property to the correct place, adds the missing enable-method to the binding documentation, and actually supports setting the "enable-method" property on multiple CPU nodes.

Without this change, "secondary-boot-reg" on even-numbered CPUs is ignored, and the value specified on the last odd-numbered CPU to be processed overrides any earlier values.

Behaviour is slightly changed by this patchset, in that the "secondary-boot-reg" property is only examined when the CPU is being enabled. This means that the omission of that property will be reported slightly later, or never if the CPU in question is never brought online. It also means that the omission in one CPU has no effect on other CPUs, whereas previously omitting it from one CPU would force the system into single-core mode.