configs/at91rm9200df: update and fix

Update to the latest maxim.org.za kernel 2.6.38
Lock down headers version, otherwise we were building a toolchain with
3.7 headers for a 2.6.33 kernel - not too wise.
Also the AT91RM9200 is an ARM920T so enable that target.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>