powerpc/mpc8572ds: Increase u-boot size to 768KB

U-boot image has grown and exceeded the predefined 512KB. Increasing to
768KB to align with other powerpc boards. Tested on MPC8572DS for 32-
and 36-bit targets with NOR flash boot. NAND boot is not covered by
this patch.

Also update board maintainer for these boards.

Signed-off-by: York Sun <yorksun@freescale.com>
Acked-by: Heiko Schocher <hs@denx.de>