rockchip: Move imply ROCKCHIP_EXTERNAL_TPL under SoC Kconfig symbol

New Rockchip SoCs will typically require use or an external TPL when
support for the SoC is added to U-Boot.

Move imply ROCKCHIP_EXTERNAL_TPL under SoC Kconfig symbol to remove a
future likelihood of a long "default y if" line.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>

arch/arm/mach-rockchip/Kconfig

@@ -178,6 +178,7 @@ config ROCKCHIP_RK3308
 	imply RNG_ROCKCHIP
 	imply ROCKCHIP_COMMON_BOARD
 	imply ROCKCHIP_COMMON_STACK_ADDR
+	imply ROCKCHIP_EXTERNAL_TPL
 	imply ROCKCHIP_OTP
 	imply SPL_CLK
 	imply SPL_DM_SEQ_ALIAS
@@ -340,6 +341,7 @@ config ROCKCHIP_RK3568
 	imply RNG_ROCKCHIP
 	imply ROCKCHIP_COMMON_BOARD
 	imply ROCKCHIP_COMMON_STACK_ADDR
+	imply ROCKCHIP_EXTERNAL_TPL
 	imply ROCKCHIP_OTP
 	imply SPL_ATF_NO_PLATFORM_PARAM if SPL_ATF
 	imply SPL_MMC_HS200_SUPPORT if SPL_MMC && MMC_HS200_SUPPORT
@@ -374,6 +376,7 @@ config ROCKCHIP_RK3588
 	imply RNG_ROCKCHIP
 	imply ROCKCHIP_COMMON_BOARD
 	imply ROCKCHIP_COMMON_STACK_ADDR
+	imply ROCKCHIP_EXTERNAL_TPL
 	imply ROCKCHIP_OTP
 	imply SCMI_FIRMWARE
 	imply SPL_ATF_NO_PLATFORM_PARAM if SPL_ATF
@@ -500,7 +503,6 @@ config TPL_ROCKCHIP_COMMON_BOARD
 
 config ROCKCHIP_EXTERNAL_TPL
 	bool "Use external TPL binary"
-	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