kinetis: Add support for LPUART module in parallel with UART module

A dispatcher function is implemented for directing writes to the correct
function. The dispatcher is bypassed completely if the CPU only contain
one kind of UART module.

There are at least two different UART hardware modules deployed in
different Kinetis CPU families (or possibly three or more when counting
variations of the UART module). The UART module is an older 8 bit module
with advanced functionality, while the LPUART is a 32 bit module with
focus on low power consumption.

 - The older families in the K series all have UART modules.
 - The K22F family have both UART and LPUART modules in the same CPU.
 - Older L series (e.g. KL25Z) have two variations of the UART module
 - Newer L series (e.g. KL43Z) have LPUART modules, and sometimes
   UART as well.
 - Newer W series (KW41Z) have only LPUART