CONFIG_MACH_ARMADA_38X is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Marvell Armada 380/385 boards

configname: CONFIG_MACH_ARMADA_38X

Linux Kernel Configuration
└─>Marvell EBU SoC variants
└─>Marvell Armada 380/385 boards
In linux kernel since version 3.10 (release Date: 2013-06-30)  
Say 'Y' here if you want your kernel to support boards based
on the Marvell Armada 380/385 SoC with device tree.