ARM: 8644/1: Reduce "CPU: shutdown" message to debug level

Similar to c68b0274 ("ARM: reduce "Booted secondary processor"
message to debug level"), demote the "CPU: shutdown" pr_notice() into a
pr_debug().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>