[POWERPC] Add of_platform support for ROM devices

This adds support for flash device descriptions to the OF device tree.
It's inspired by and partially borrowed from Sergei's patch "[RFC]
Adding MTD to device tree.patch".

Signed-off-by: Vitaly Wool <vwool@ru.mvista.com>
Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>