serial: lpuart: Add driver model serial support

This adds driver model support to lpuart serial driver.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Tested-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>