drivers: counter: increase COUNTER_INIT_PRIORITY

Some counter devices depend on buses (e.g. I2C for Maxim DS3231). Bus
devices are tipically initialized with KERNEL_INIT_PRIORITY_DEVICE as
well, so there's no guarantee counter will be initialized be initialized
before without manually tweaking priorities.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>