(MESS) ti85: add basic support for the TI-84 Plus

Adds known dump of TI-84 Plus which has been sent to a mess dev and supports
enough asic functionality that it boots and runs.

Fixed up model enum to match style from the apple2 and used it to add support
for where the ti84p differs from the ti84pse.

Consolidated code for the 15Mhz calculators so there is less duplicated code
between the ti83pse, ti84pse and ti84 support code.