Input: cros_ec_keyb - add an EC event for sysrq

Some form factors (detachables/tablets) may not have a keyboard and
thus user may have to resort to using a defined EC UI to send sysrq(s)
to the kernel in order to collect crash info etc. This UI typically
is in the form of user pressing volume / power buttons in some specific
sequence and for some specific time. Add a new EC event that allows EC
to communicate the sysrq to the AP.

(We're skipping event number 5 because it has been reserved for
something else)

Signed-off-by: Rajat Jain <rajatja@google.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>