ACPI: update CONFIG_ACPI_PROCFS description

Update CONFIG_ACPI_PROCFS description because the processor,
video and thermal zone procfs I/F have been removed.

Some ACPI drivers, e.g. button, have their procfs I/F always built in,
because we don't have sysfs I/F replacement at the moment.
But once we finish developing the sysfs I/F for these driver,
we need CONFIG_ACPI_PROCFS to enabled/disable the corresponding procfs I/F.

So just updating the description rather than removing this option,
although there is no procfs I/F depends on it for now.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>