[ARM] Kirkwood: Add MPP44 (board ID) on TS-219

MPP44 can be used to differentiate between one-bay (TS-11x) and
two-bay (TS-21x) devices.

According to an engineer from QNAP, the setting of MPP44 depends
on the firmware rather than hardware.  Presumably, this means
that you could fake the MPP44 value by changing the boot loader.

Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>