rockchip: Kconfig: Enable external TPL binary for rk3308

rockchip: Kconfig: Enable external TPL binary for rk3308

There is no support to initialize DRAM on rk3308 SoC using U-Boot
TPL or SPL, and therefore an external TPL binary must be used to
package a bootable u-boot-rockchip.bin image.

Default ROCKCHIP_EXTERNAL_TPL to yes if ROCKCHIP_RK3308.
Remove useless TPL_SERIAL.

Signed-off-by: Massimo Pegorer <massimo.pegorer+oss@gmail.com>
Tested-by: FUKAUMI Naoki <naoki@radxa.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>