drivers: serial: nrf: Use nrfx GPIO HAL to properly handle pins from P1

The dependency on the GPIO driver was a bit artificial, since the UART
driver used the GPIO API functions only to configure the pins at its
initialization. This configuration will be done by using nrfx GPIO HAL
functions directly, just like in case when a shim is using
an nrfx driver underneath.

HAL functions can be fed with the absolute pin numbers, that are
currently used for Nordic SoCs in Kconfig and DT, and for the GPIO
driver an additional translation would be required, to extract
the port and its relative pin number.

UART driver has to be initialized at PRE_KERNEL_1 (because of the
console using it) and currently the GPIO driver is initialized
at POST_KERNEL stage, so calling its API function form the UART
initialization routine was incorrect.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>