ARM: reduce "Booted secondary processor" message to debug level

Drop the "CPUn: Booted secondary processor" message from info to debug
level.  We later print how many CPUs came online, so listing each one
is redundant, and when using hotplug, can be quite noisy.

Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>