CONFIG_OF_RTC is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_OF_RTC (not configurable)

modulename: of_rtc.ko

configname: CONFIG_OF_RTC

Linux Kernel Configuration
└─>Platform support
└─>OF_RTC
In linux kernel since version 2.6.25 (release Date: 2008-04-16)  
Uses information from the OF or flattened device tree to instatiate
platform devices for direct mapped RTC chips like the DS1742 or DS1743.

source code:
arch/powerpc/sysdev/of_rtc.c