PowerPC is the last architecture using the GEN_RTC driver on some
machines, but we can migrate them all to using the RTC_DRV_GENERIC
driver instead now.
This moves over the CONFIG_GEN_RTC option from drivers/char into
arch/powerpc/platforms/Kconfig and makes it just select the
replacement driver instead, for the only reason of not breaking
existing defconfig and .config files that users may have.