mtd-ram: add of-support

[Wolfram Sang]

Hello,

as we needed an of-mtd-ram driver to support SRAM on the
phyCORE-MPC5200B-IO, I did the following:

Patch 1:
  refactor plat-ram.c to have generic probe and remove
  routines

Patch 2:
  use the new routines for an of-driver

Looking forward to comments. (Sidenote: Converting all these platform
drivers to support of is really tiring, sigh. Isn't there any other
way?)

Kind regards,

   Wolfram