ARM64: Use pr_debug() for CPU hotplug messages

Currently, CPU online messages are printed using printk(),
and CPU offline messages are printed using pr_notice().
It seems rather silly to have the (otherwise symmetric)
messages to be printed using different log levels. Change
both messages to be printed using pr_debug() to match the
ARM implementation, to avoid disparities in log spam on
ARM/ARM64 with otherwise identical loglevel settings.

Change-Id: I39103f245ba0b5904d4c02fa0c67f89c5058523b
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>