drivers: i2c: Refactor drivers to use shared init priority

Refactors the remaining I2C drivers to use the shared driver class
initialization priority configuration, CONFIG_I2C_INIT_PRIORITY, to
allow configuring I2C drivers separately from other devices. This is
similar to other driver classes.

The default is set to CONFIG_KERNEL_INIT_PRIORITY_DEVICE to preserve the
existing default initialization priority for most drivers.

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