CONFIG_CPU_SUBTYPE_SH7751 is not available for thedefaultarchitecture x86.
Result is shown for architecture sh

SH7751/SH7751R

modulename: setup-sh7750.ko

and/or

pci-sh7751.ko

configname: CONFIG_CPU_SUBTYPE_SH7751

Processor subtype
└─>SH7751/SH7751R
In linux kernel since version 2.6.12  
Select SH7751 if you have a 166 Mhz SH-4 HD6417751 CPU,
or if you have a HD6417751R CPU.

source code:
arch/sh/drivers/pci/pci-sh7751.c
selects

CONFIG_CPU_SH4

depends
CONFIG_CPU_SH4