drivers: counter: Refactor drivers to use shared init priority

Refactors all of the counter drivers to use a shared driver class
initialization priority configuration, CONFIG_COUNTER_INIT_PRIORITY, to
allow configuring counter drivers separately from other devices. This is
similar to other driver classes like I2C and SPI.

The default is set to CONFIG_KERNEL_INIT_PRIORITY_DEVICE to preserve the
existing default initialization priority for most drivers. The
exceptions are the maxim_ds3231 and mcp7940n drivers which have a
dependency on a SPI driver and must therefore initialize later than the
default device priority.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>