(MESS) a7800: added Yamaha 2151 emulation to the XM
expansion board, thanks to info from tep392. nw.

Also removed from the softlist the Donkey Kong XM Enhanced 
version which had been hacked to work on ProSystem emulator
and fails to save hiscores with correct HSC emulation 
(it remains available the correct version which saves fine both
in MESS and in the real hardware).