driver: npcx: i2c: set a dedicated priority for the port driver

The NPCX I2C controller has a port-controller hierarchy and the driver
is split in two files, with separate device struct and init functions.

These are currently initialized at the same level and priority, so the
actual order depends on what the linker does.

To avoid relying on the linking order, add a dedicated priority option
for the port that is set to go after the normal I2C one by default.

Found this by building with CONFIG_CHECK_INIT_PRIORITIES.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>