serial: Introduce new mcux shim driver

Adds a shim layer around the mcux uart driver to adapt it to the Zephyr
serial interface. Unlike the existing (and confusingly-named) uart_k20
driver, this driver can be used for k64 and other Kinetis SoCs.

Implements polling and interrupt-driven serial interface functions that
are logically equivalent to the uart_k20 driver. Adds an extra instance,
irq_tx_empty(), and err_check() that aren't implemented in the uart_k20
driver.

Jira: ZEP-719
Change-Id: Iab99542e7ec921ef4f361437768113fee01e5fe8
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>