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

CONFIG_MXS_HAVE_AMBA_DUART (not configurable)

modulename: amba-duart.ko

configname: CONFIG_MXS_HAVE_AMBA_DUART

Linux Kernel Configuration
└─>MXS_HAVE_AMBA_DUART
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  
source code:
arch/arm/mach-mxs/devices/amba-duart.c
selects
CONFIG_ARM_AMBA