[ARM] Kirkwood: Add MPP36 for QNAP TS-11x/TS-21x

MPP36 is used on the QNAP TS-11x/TS-21x devices to indicate how
much RAM there is: it's high for 512 MB RAM (TS-x19) and low for
256 MB (TS-x10).  While this may not be very useful, let's add
it for completeness.

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