CONFIG_SOC_AT91SAM9G45 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

AT91SAM9G45 or AT91SAM9M10 families

modulename: at91sam9g45.ko

configname: CONFIG_SOC_AT91SAM9G45

Linux Kernel Configuration
└─>Atmel AT91 System-on-Chip
└─>AT91SAM9G45 or AT91SAM9M10 families
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Select this if you are using one of Atmel's AT91SAM9G45 family SoC.
This support covers AT91SAM9G45, AT91SAM9G46, AT91SAM9M10 and AT91SAM9M11.

source code:
arch/arm/mach-at91/at91sam9g45.c