clocksource/drivers/timer-microchip-pit64b: Select CONFIG_TIMER_OF

This driver is an OF driver, it depends on OF, and uses
TIMER_OF_DECLARE, so it should select CONFIG_TIMER_OF.

Without CONFIG_TIMER_OF enabled this can lead to warnings such as:

  powerpc-linux-ld: warning: orphan section `__timer_of_table' from
  `drivers/clocksource/timer-microchip-pit64b.o' being placed in
  section `__timer_of_table'.

Because TIMER_OF_TABLES in vmlinux.lds.h doesn't emit anything into
the linker script when CONFIG_TIMER_OF is not enabled.

Fixes: 625022a5 ("clocksource/drivers/timer-microchip-pit64b: Add Microchip PIT64B support")
Cc: stable@vger.kernel.org # v5.6+
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200426124356.3929682-1-mpe@ellerman.id.au