CONFIG_DEBUG_BCM_5301X is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging on BCM5301X UART1

configname: CONFIG_DEBUG_BCM_5301X

Kernel low-level debugging port
└─>Kernel low-level debugging on BCM5301X UART1
In linux kernel since version 3.15 (release Date: 2014-06-08)  

depends
CONFIG_ARCH_BCM_5301X or | or CONFIG_ARCH_BCM_NSP