ar71xx-generic: fix DIR825C1

Build failed with:
 * unable to set 'CONFIG_TARGET_DEVICE_PACKAGES_ar71xx_generic_DEVICE_dir-825-c1
 * unable to enable device 'dir-825-c1'

This was likely because the device is defined in
target/linux/ar71xx/image/generic-legacy-devices.mk as DIR825C1 and
hence needs 'profile' to be set.