ARC: timer: gfrc, rtc: build under same option (64-bit timers)

The original distinction was done as they were developed at different
times and primarily because they are specific to UP (RTC) and SMP (GFRC).

But given that driver handles that at runtime, (i.e. not allowing
RTC as clocksource in SMP), we can simplify things a bit.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>