powerpc: clean up DIU macro definitions for Freescale reference boards

Clean up the macro defintions used to enable DIU (video) support on the
MPC8610HPCD and the MPC5121ADS so that they look more like the P1022DS,
which is newer.  Add software cursor support to all three boards.

Also document the CONFIG_FSL_DIU_FB in the README.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>