ARM: dts: kirkwood: Fix Openblock A6 nand partition overlap

The "test" and "conf" nand partitions partly overlap:

Creating 6 MTD partitions on "orion_nand":
0x000000000000-0x000000090000 : "uboot"
0x000000090000-0x0000000d4000 : "env"
0x0000000d4000-0x0000000f8000 : "test"
0x0000000f4000-0x0000004f4000 : "conf"

That is unlikely to be desired, and not matching the partition map used in
u-boot - So adjust the test partition size to fix this.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>