ACPI / sysfs: Use new GPE masking mechanism in GPE interface

Now GPE can be masked via the new acpi_mask_gpe() API and this patch
modifies /sys/firmware/acpi/interrupts/gpexx to use this new facility.

Writes "mask/unmask" to this file now invokes acpi_mask_gpe().

Reads from this file now returns new "EN/STS" when the corresponding GPE
hardware register's EN/STS bits are flagged, and new "masked/unmasked"
attribute to indicate the status of the masking mechanism.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>