hwdb: Map 8a to f20 on the Acer Travelmate P648-G3-M

This model emits 0x8a for the microphone-mute button above the keyboard,
so let's map it to correct keycode.

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: João Paulo Rechi Vita <jprvita@endlessm.com>