clocksource: fix compilation if no GENERIC_TIME

clocksource: fix compilation if no GENERIC_TIME

commit a362c638bdf052bf424bce7645d39b101090f6ba upstream

Commit a9238ce3bb0fda6e760780b702c6cbd3793087d3 broke compilation on
platforms that do not implement GENERIC_TIME (e.g. iop32x):

  kernel/time/clocksource.c: In function 'clocksource_register':
  kernel/time/clocksource.c:556: error: implicit declaration of function 'clocksource_max_deferment'

Provide the implementation of clocksource_max_deferment() also for
such platforms.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>