CONFIG_MACH_DECSTATION is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

DECstations

configname: CONFIG_MACH_DECSTATION

System type
└─>DECstations
In linux kernel since version 2.6.12  
This enables support for DEC's MIPS based workstations. For details
see the Linux/MIPS FAQ on <http://www.linux-mips.org/> and the
DECstation porting pages on <http://decstation.unix-ag.org/>.

If you have one of the following DECstation Models you definitely
want to choose R4xx0 for the CPU Type:

DECstation 5000/50
DECstation 5000/150
DECstation 5000/260
DECsystem 5900/260

otherwise choose R3000.