configs: add qcm6490_defconfig

Introduce a defconfig for the RB3 Gen 2 and other QCM6490 boards with a
dedicated uefi partition. These can replace EDK2 entirely with U-Boot.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>