hwdb: Add mapping for Xiaomi Mipad 2 bottom bezel capacitive buttons

hwdb: Add mapping for Xiaomi Mipad 2 bottom bezel capacitive buttons

The Xiaomi Mipad2 tablet has 3 capacitive buttons at the bottom.
These 3 buttons are handled by the touchsceen (the touchscreen extends
downwards to cover the bottom bezel).

By default these send Windows specific shortcuts which do not work
with Linux desktop environments:

Menu: LeftMeta + S
Home: LeftCtrl + Esc
Back: Backspace

Map these to menu / leftmeta (Windows key / Win 8 tablet home button key) /
back.