CONFIG_MACH_ARMADA_39X is not available for the default architecture x86.
Result is shown for architecture arm

Marvell Armada 39x boards

configname: CONFIG_MACH_ARMADA_39X

Linux Kernel Configuration
└─> Marvell Engineering Business Unit (MVEBU) SoCs
└─> Marvell Armada 39x boards

Say 'Y' here if you want your kernel to support boards based
on the Marvell Armada 39x SoC with device tree.