arm/imx21: Framebuffer support for i.MX21

This patch mimicks what Martin wrote on the mailing list:

* move arch/arm/mach-imx/include/mach/imxfb.h into
  arch/arm/mach-mxc/include/mach/imxfb.h
* changes Kconfig so that CONFIG_FB_IMX is selectable
* adds a platform device (copied from some pengutronix
  patches)

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>