rtc: brcmstb-waketimer: Set wktmr prescaler

The HW default is one tick per second, however instead of assuming this,
lets make sure the waketimer is actually one tick per second before
arming the alarm.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>