HID: input: add mapping for Expose/Overview key

According to HUTRR77 usage 0x29f from the consumer page is reserved for
the Desktop application to present all running user’s application windows.
Linux defines KEY_SCALE to request Compiz Scale (Expose) mode, so let's
add the mapping.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>