drivers: serial: nrfx_uart: add support for pinctrl

This patch adds support for the new pinctrl API to the UART driver. The
old pin property based solution is still kept so that users have time to
transition to the new model.

Notes:

- A new property to disable RX has been introduced: disable-rx. It is no
  longer possible to do it automatically depending on pin information,
  since it's not available when using pinctrl.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>