[Saucy SRU] Add new keycode to dell-wmi

[Alex Hung]

SRU Justification: 

Impact: New Dell laptop supports a new mic mute hotkey. Pressing the hotkey
	generates a BIOS event which needs to be converted to a keycode 

Fix:	Add a keycode to bios_to_linux_keycode table in dell-wmi

Note:

The commit is cfb743bf6173063b57ef5a8185ea87f130209d4d in upstream kernel

BugLink: http://bugs.launchpad.net/bugs/1257268