greybus: add support for the log protocol

Add support for the new Log class/protocol. This protocol allows modules
to send their internal logging messages to the AP in order to make
module debugging easier.

The protocol is, for now, composed a single module-initiated request.
This request contains a message and associated length. The message is
integrated in the kernel log with dev_dbg(). In order to be displayed
with 'dmesg', the following command needs to be entered first:

$ echo "file log.c +p" > /sys/kernel/debug/dynamic_debug/control

The major portion of this file was initially written by Greg KH.

Signed-off-by: Joel Porquet <porquet_joel@projectara.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>