OMAP3: overo: increase linux partition to 8 MiB

Linux kernel at version 3.5 is about 3.5 MiB; test kernels for 3.10 exceed 4
MiB.  Prepare for future upgrades by increasing the NAND partition now.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Tested-by: Ash Charles <ash@gumstix.com>