ARM: BCM: modify Broadcom CPU 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 actually support multiple "secondary-boot-reg"
properties.

This patchset rectifies that omission. Note that the behaviour is changed
slightly in that the "secondary-boot-reg" property is now read in
smp_boot_secondary() rather than smp_prepare_cpus(). This means that any
omissions will now only be reported when and if the cpu in question is being
brought up. It also means that an omission for one cpu will not force
uniprocessor mode.

Signed-off-by: Chris Brand <chris.brand@broadcom.com>
Reviewed-and-Tested-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>