mpc83xx: make it bootable with the latest kernel

Due to the blow up of the latest kernel size, the default gnuzip
size (8M) seems too small. The yocto kernel size I built for
mpc8315erdb board is 5294393, and it can't be boot by using the
latest u-boot. So expand gnuzip buffer for all the mpc83xx boards
to fix this issue.

Robert P. J. Day also pointed that the kernel partition on the NAND
flash is also too small, fix it at the same time.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Reviewed-by: York Sun <york.sun@nxp.com>