ARM: imx_v6_v7_defconfig: Select CONFIG_HIGHMEM

Booting a mx6q wandboard with 2GB of RAM we see the following on boot:

Booting Linux on physical CPU 0x0
Linux version 3.12.0-next-20131112+ (fabio@fabio-Latitude-E6410) (gcc version 43
CPU: ARMv7 Processor [412fc09a] revision 10 (ARMv7), cr=10c53c7d
CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing instruction cache
Machine model: Wandboard i.MX6 Quad Board
Truncating RAM at 10000000-8fffffff to -7f7fffff (vmalloc region overlap)
...

Select CONFIG_HIGHMEM to avoid the vmalloc region overlap.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>