ARM: kirkwood: enable HIGHMEM in defconfig

The Kirkwood-based OpenBlocks A7 platform has 1 GB of memory. To make
use of this memory completely, enabling HIGHMEM is needed, so we
enable it by default in the kirkwood_defconfig.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>