CONFIG_MACH_RD88F6281 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_MACH_RD88F6281.
Result is shown for kernelversion 6.5.1

Marvell RD-88F6281 Reference Board

modulename: rd88f6281-setup.ko

configname: CONFIG_MACH_RD88F6281

Linux Kernel Configuration
└─>Marvell Kirkwood Implementations
└─>Marvell RD-88F6281 Reference Board
In linux kernel since version 2.6.27 (release Date: 2008-10-09)  
Say 'Y' here if you want your kernel to support the
Marvell RD-88F6281 Reference Board.

source code:
arch/arm/mach-kirkwood/rd88f6281-setup.c