configs: rockchip: add DOS_PARTITION to RK3308 boards defconfig

Without DOS_PARTITION support U-Boot is not able to boot an OS stored
into an SD card with MBR partitions table. This is still a quite common
case so add DOS_PARTITION (only for U-Boot proper build) to Rockchip
RK3308 EVB, Radxa ROCK Pi S and Firefly roc-rk3308-cc boards: they are
the only RK boards missing of DOS_PARTITION.

Reported-by: Jayantajit Gogoi <jayanta.gogoi525@gmail.com>
Signed-off-by: Massimo Pegorer <massimo.pegorer+oss@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>