video: fsl_dcu_fb: fix framebuffer to the end of memory

Fix the framebuffer location to the very end of the available memory.
This allows to remove the area from available memory for the kernel,
which in turn allows to display the splash screen through the Linux
kernel boot process.

Ideas has been taken from the sunxi display driver, e.g.
20779ec3 ("sunxi: video: Dynamically reserve framebuffer memory")

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>