ARM: dts: mmp2: add OLPC XO 1.75 machine

This is a fairly complete description of an OLPC XO 1.75 laptop.
What's missing for now is the GPU, LCD controller, DCON, the panel and
audio.

The machine is booted with OpenFirmware and thus has a devicetree.
However, older versions are unable to create a valid FDT and don't
follow the Linux bindings. Having an device tree in the kernel tree
makes it easier to use mainline kernels on such machines, test changes
with CONFIG_ARM_APPENDED_DTB and give a good reference on what bindings
are used on the machine without an access to one.

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