ARM: dts: mmp2: add more TWSI controllers

I've gotten the base addresses, clocks and interrupts from an rusty and old
out-of-tree driver. I haven't actually checked against the datasheet, since
that one is reserved for the Marvell inner circle.

Tested with an accelerometer on TWSI6 on an OLPC XO 1.75 machine.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Olof Johansson <olof@lixom.net>