ARM: dts: mmp2: specify reg-shift for the UARTs

This makes the 8250_of driver happy. There are two more drivers in the
tree that bind to mrvl,mmp-uart compatibles: pxa and 8250_pxa and
neither of them requires the reg-shift property, assuming it's always 2.

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