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>