ARM: dts: omap3-gta04: make NAND partitions compatible with recent U-Boot

Vendor defined U-Boot has changed the partition scheme a while ago:

* kernel partition 6MB
* file system partition uses the remainder up to end of the NAND
* increased size of the environment partition (to get an OneNAND compatible base address)
* shrink the U-Boot partition

Let's be compatible (e.g. Debian kernel built from upstream).

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>