configs: rockchip: rk3308: use CONFIG_DEFAULT_FDT_FILE

all rk3308 boards should use their own dtb file.

also, change fdt_addr_r to avoid following error:
 "ERROR: Did not find a cmdline Flattened Device Tree"
it happens on Radxa ROCK Pi S (256MB/512MB) with kernel built from
Radxa BSP.

Signed-off-by: FUKAUMI Naoki <naoki@radxa.com>