ACPI: Provide /sys/kernel/debug//ec/ec0/io for binary access to the EC

A userspace app to easily read/write the EC can be found here:
ftp://ftp.suse.com/pub/people/trenn/sources/ec/ec_access.c

Multiple ECs are not supported, but shouldn't be hard to add as soon
as the ec driver itself will support them.

Signed-off-by: Thomas Renninger <trenn@suse.de>
CC: Alexey Starikovskiy <astarikovskiy@suse.de>
CC: Len Brown <lenb@kernel.org>
CC: linux-kernel@vger.kernel.org
CC: linux-acpi@vger.kernel.org
CC: platform-driver-x86@vger.kernel.org
Signed-off-by: Matthew Garrett <mjg@redhat.com>