CONFIG_SH_CUSTOM_CLK is not available for thedefaultarchitecture x86.
Result is shown for architecture sh

CONFIG_SH_CUSTOM_CLK (not configurable)

configname: CONFIG_SH_CUSTOM_CLK

Linux Kernel Configuration
└─>Board support
└─>SH_CUSTOM_CLK
In linux kernel since version 5.1 (release Date: 2019-05-05)  

depends
NOT CONFIG_SH_DEVICE_TREE