CONFIG_DEBUG_RK3X_UART3 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging messages via Rockchip RK30/RK31 UART3

configname: CONFIG_DEBUG_RK3X_UART3

Kernel low-level debugging port
└─>Kernel low-level debugging messages via Rockchip RK30/RK31 UART3
In linux kernel since version 3.11 (release Date: 2013-09-02)  
Say Y here if you want kernel low-level debugging support
on Rockchip based platforms.