ARM: dts: mmp2: fix the SPI nodes

The SPI bus has a single address cell and not size cells.

Also, dtc thinks the SPI nodes are preferrably called "spi" and it is
right to think so.

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