CONFIG_DEBUG_RK3X_UART0 is not available for the selected architecture x86.
Result is shown for architecture arm

RK3X UART0

configname: CONFIG_DEBUG_RK3X_UART0

Low-level debug console UART
└─>RK3X UART0
In linux kernel since version 3.11 (release Date: 2013-09-02)
selects
CONFIG_DEBUG_UART_8250

depends
 or CONFIG_ARCH_ROCKCHIP