Documentation: Binding docs for bcm11351 enable method

Commit 84320e1a
("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 update brcm,bcm11351-cpu-method.txt to match.

bcm11351-cpu-method was apparently never added to the list of methods in
bindings/arm/cpus.txt.

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

This patch rectifies these two omissions and one inconsistency.

Signed-off-by: Chris Brand <chris.brand@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>