drivers: serial: nrfx_uarte: add support for pinctrl

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

Notes:

- Some build assertions cannot be performed since the driver does not
  have direct access to pin settings anymore. As a result user will not
  be notified if HWFC is enabled but RTS/CTS pins are not configured.
- Hardware flow control can be enabled regardless of pin configuration,
  it is now up to the user to configure RTS/CTS pins in DT.
- Some RX enable checks that were performed using pin information has
  been replaced with a DT property that informs if RX is enabled or not.

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