CONFIG_CPU_SUBTYPE_SH7091 is not available for thedefaultarchitecture x86.
Result is shown for architecture sh

Support SH7091 processor

modulename: setup-sh7750.ko

configname: CONFIG_CPU_SUBTYPE_SH7091

Processor sub-type selection
└─>Support SH7091 processor
In linux kernel since version 2.6.16 (release Date: 2006-03-19)  
Select SH7091 if you have an SH-4 based Sega device (such as
the Dreamcast, Naomi, and Naomi 2).

source code:
arch/sh/kernel/cpu/sh4/setup-sh7750.c
selects
CONFIG_CPU_SH4

is selected by
CONFIG_SH_DREAMCAST