clocksource/drivers/timer-microchip-pit64b: Fix rate for gck

Generic clock rate needs to be set in case it was selected as timer clock
source in mchp_pit64b_init_mode(). Otherwise it will be enabled with wrong
rate.

Fixes: 625022a5

 ("clocksource/drivers/timer-microchip-pit64b: Add Microchip PIT64B support")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1584352376-32585-1-git-send-email-claudiu.beznea@microchip.com