ARM: 6055/1: AT91: add defconfig for taskit PortuxG20 and Stamp9G20

Support for both boards can easily be built into one kernel image, so
there is no need to add one defconfig for each of them.
Signed-off-by: Christian Glindkamp <christian.glindkamp@taskit.de>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>