ARM: at91: udpate defconfigs

Merge 9g20 with 9260 and 9g10 with 9261 as those SoCs can run from the same
kernel even in non DT world.

Fix the sam9261ek to allow 9g10 and sam9261 to compile together.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>