hwdb: add XKB_FIXED_MODEL to the keyboard hwdb

Chromebook keyboards have a top row which generates f1-f10 key codes but
the keys have media symbols printed on them. A simple scan code to key
code mapping to the correct media keys makes the f1-f10 inaccessible. To
properly use the keyboard a custom key code to symbol mapping in xbk is
required (a variant of the chromebook xkb model is already upstream).
Other devices have similar problems.
This commit makes it possible to specify which xkb model should be used
for a specific device by setting XKB_FIXED_MODEL.