ARM: imx: platform-imx-fb: modifies platform device name

Framebuffer platform device is now identified by a device id (imx1-fb or
imx21-fb) instead of by driver name (imx-fb).

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@armadeus.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>