printk: Log printk events

Log printk events to the uncached buffer. If a reset occurs
and printk data is still present in the cache, information will
be lost. This doesn't store the actual printk, but it at least
gives information on who the last caller was.

Change-Id: If1bd414a44b19b45c157d23a7ddde8f16b30e780
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Trevor Bourget <tbourget@codeaurora.org>