The genrtc driver serves no purpose on Alpha because it drives the
same hardware as the original rtc.c driver, and the newer rtc-generic.c
or rtc-cmos.c drivers on architectures that use the asm-generic/rtc.h
header.
The defconfig uses CONFIG_RTC=y, so this driver is not used by default.
At one point it was used to abstract a quirk for the "Marvel" platform,
but it does not do this any more after the code was moved into yet
another driver in arch/alpha/kernel/rtc.c.