drivers/video: fsl-diu-fb: improve device tree usage

Implement various improvements to the way the Freescale DIU framebuffer
driver access the device tree.

1) Use of_iomap() instead of of_address_to_resource() and ioremap()

2) Use be32_to_cpup() instead of directly dereferencing the device_node
   pointer.

3) Rename variable 'ofdev' to 'pdev' to avoid any confusion that it's
   a platform_device pointer, not an of_device pointer (of_device no
   longer exists).

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>