[POWERPC] Don't link timer.o for powerpc systems using generic rtc

[POWERPC] Don't link timer.o for powerpc systems using generic rtc

With both generic rtc and powerpc timer suspend / resume code now in the
(powerpc.git) tree, powerpc platforms using the generic timer and enabling
power management will have timer.o linked in the kernel, which they don't
need. Moreover, it will likely WARN_ON(!ppc_md.get_rtc_time), save
zero-time and return no error on suspend...

As a possible solution we can choose not to build timer.o when RTC_CLASS
is enabled.  However, I can imagine systems with 2 rtc's, one served by the
ppc-rtc, another one generic built as a module, in which case using the
ppc-rtc for suspend / resume will be impossible.  Not to say, that such a
configuration would be ugly...

Signed-off-by: G. Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>