x86/olpc: Add debugfs interface for EC commands

Add a debugfs interface for sending commands to the OLPC
Embedded Controller (EC) and reading the responses. The EC
provides functionality for machine identification, battery and
AC control, wakeup control, etc.

Having a debugfs interface available is useful for EC
development and debugging.

Based on code by Paul Fox (who also approves of the end result).

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Paul Fox <pgf@laptop.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andres Salomon <dilinger@queued.net>
Link: http://lkml.kernel.org/r/20120327150740.667D09D401E@zog.reactivated.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>