drivers: serial: Configurable GPIO management in NRFX UARTE

In some applications where power management is used, it is not expected
that the GPIO pins of the UARTE is reconfigured upon power state change.
Added a Kconfig option to disable the UARTE driver management of GPIOs.
It defaults to y in order to not affect current behavior unless the user
configures it specifically.

Signed-off-by: Dennis Wildmark <dennis.wildmark@assaabloy.com>