mpc85xx/kmp204x: raise u-boot size to 768KB

mpc85xx/kmp204x: raise u-boot size to 768KB

Until now this defined to be 512KB and the total binary size actually
was on the edge of this limit. Most of the powerpc boards have thus
moved to 768KB.

Since on the current kmp204x boards there is 1MB reserved for u-boot on
the SPI boot flash, there is no problem to set the limit to 768KB as
well to be on line with the other powerpc boards and to eventually
configure in some additional features (and binary size) to u-boot.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Reviewed-by: York Sun <yorksun@freescale.com>