CONFIG_MACH_ASPEED_G6 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Aspeed SoC 6th Generation

modulename: clk-ast2600.ko

configname: CONFIG_MACH_ASPEED_G6

Linux Kernel Configuration
└─>Aspeed BMC architectures
└─>Aspeed SoC 6th Generation
In linux kernel since version 5.1 (release Date: 2019-05-05)  
Say yes if you intend to run on an Aspeed ast2600 or similar
sixth generation Aspeed BMCs.

source code: