removes the only reference to atrtc_set() from outside of atrtc.c, so make
it static.

The xen timer driver registers as a realtime clock with 1us resolution.  In
the past that resulted in only the xen timer's clock_settime() getting
called, so it would call atrtc_set() to set the hardware clock as well.  As
of r32090, the clock_settime() method of all registered realtime clocks gets
called, so the xen driver no longer needs to chain-call the lower-resolution
driver.

Thanks to royger@ for talking me through the xen stuff, and for testing.