keymap: Add microphone mute keymap for Dell Latitude

On the Dell Latitude, the mic mute key event is generated by wmi
driver, the keycode assigned to this hotkey from kernel is
KEY_MICMUTE (248), this keycode is too big for xorg to handle,
in the xorg, the XF86AudioMicMute is assigned to F20.

Please refer to 4e648ea0 of xkeyboard-config.

BugLink: https://bugs.launchpad.net/bugs/1326684
BugLink: https://bugs.launchpad.net/bugs/1339998
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>