fsl_diu_fb: further refactoring of FSL DIU code

Move common code to the fsl_diu_fb.c file and remove obsolete
code from board files (aria, mpc8610hpcd and pdm360ng).
Move fsl_diu_fb.h file to the include directory.

Signed-off-by: Anatolij Gustschin <agust@denx.de>