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>
diff --git a/arch/arm/mach-rockchip/Kconfig b/arch/arm/mach-rockchip/Kconfig
index 03c2b37..c43c185 100644
--- a/arch/arm/mach-rockchip/Kconfig
+++ b/arch/arm/mach-rockchip/Kconfig
@@ -166,7 +166,6 @@
 	imply SPL_SYSCON
 	imply SPL_RAM
 	imply SPL_SERIAL
-	imply TPL_SERIAL
 	imply SPL_SEPARATE_BSS
 	help
 	  The Rockchip RK3308 is a ARM-based Soc which embedded with quad
@@ -436,7 +435,7 @@
 
 config ROCKCHIP_EXTERNAL_TPL
 	bool "Use external TPL binary"
-	default y if ROCKCHIP_RK3568 || ROCKCHIP_RK3588
+	default y if ROCKCHIP_RK3308 || ROCKCHIP_RK3568 || ROCKCHIP_RK3588
 	help
 	  Some Rockchip SoCs require an external TPL to initialize DRAM.
 	  Enable this option and build with ROCKCHIP_TPL=/path/to/ddr.bin to