powerpc/85xx: clean up P1022DS board configuration header file

Remove some unused default environment variables (memctl_intlv_ctl,
perf_mode, diuregs, dium, and diuerr), update 'tftpflash' variable,
and add videobootargs as a Linux command line variable (so that we can
easily pass video= to the kernel).

Signed-off-by: Timur Tabi <timur@freescale.com>