keymap: remap microphone mute keycode for Lenovo Thinkcentre M800z

keymap: remap microphone mute keycode for Lenovo Thinkcentre M800z

This Lenovo machine use codec Line2 to implement a microphone mute
button, it depends on the unsolicited interrupt to generate key event,
the scan code for this button is assigned to 0x00 in the linux kernel
driver, and the keycode is KEY_MICMUTE(248), we need to remap this
keycode to KEY_F20 to make this hotkey work in X11.

BugLink: https://bugs.launchpad.net/bugs/1531362
Signed-off-by: Hui Wang <hui.wang@canonical.com>