ACPI / tools: Introduce ec_access.c - tool to access the EC

This userspace tool accesses the EC through the ec_sys debug driver
(through /sys/kernel/debug/ec/ec0/io).

The EC command/data registers cannot be accessed directly, because they
may be manipulated by the AML interpreter in parallel.

The ec_sys driver synchronizes user space (debug) access with the AML
interpreter.

Signed-off-by: Thomas Renninger <trenn@suse.de>
[rjw: Changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>