nRF5: enable UARTE peripheral support

[mike-scott]

The currently supported nRF52832 and nRF52840 boards have the option of using UARTE0 peripheral instead of UART0. This provides better power usage due to EasyDMA use.

On nRF52840 there is also a UARTE1 peripheral.

This issue tracks efforts to enable UARTE.

[jakub-uC]

PR8640 will provide UARTE0 and UARTE1 driver.

[mike-scott]

@jarz-nordic: its on my TODO list :)

[carlescufi]

Completed