ACPI / PM: Generate wakeup events on fixed power button

When the system is woken up by the ACPI fixed power button, currently there
is no way of userspace becoming aware that the power button was pressed.

OLPC would like to know this, so that we can respond appropriately.
For example, if the system was woken up by a network packet, we know
we can go back to sleep very quickly. But if the user explicitly woke the
system with the power button, we're going to want to stay awake for a
while.

The wakeup count mechanism seems like a good fit for communicating this.
Mark the fixed power button as wakeup-enabled, and increment its wakeup
counter when the system is woken with the power button. (The wakeup counter
is also incremented when the power button is pressed during system
operation; this is already handled by an existing acpi-button codepath).

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>