drivers: gpio: nrfx: add custom drive flags

Now that we have 8 bits reserved for vendor specific GPIO flags,
introduce a new set of flags for nRF platforms to configure pins drive
mode. These new flags are equivalent to the previous existing ones, but
use a naming scheme the fits better with vendor hardware capabilities.

The table below shows the equivalence between old and new flag

| Old flags                 | New flags             |
|---------------------------|-----------------------|
| `NRF_GPIO_DS_DFLT_LOW`    | `NRF_GPIO_DRIVE_S0`   |
| `NRF_GPIO_DS_DFLT_HIGH`   | `NRF_GPIO_DRIVE_S1`   |
| `NRF_GPIO_DS_ALT_LOW`     | `NRF_GPIO_DRIVE_H0`   |
| `NRF_GPIO_DS_ALT_HIGH`    | `NRF_GPIO_DRIVE_H1`   |
| `NRF_GPIO_DS_DFLT`        | `NRF_GPIO_DRIVE_S0S1` |
| `NRF_GPIO_DS_ALT`         | `NRF_GPIO_DRIVE_H0H1` |
| `NRF_GPIO_DS_DFLT_LOW \|` | `NRF_GPIO_DRIVE_S0H1` |
| `NRF_GPIO_DS_ALT_HIGH`    |                       |
| `NRF_GPIO_DS_ALT_LOW \|`  | `NRF_GPIO_DRIVE_H0S1` |
| `NRF_GPIO_DS_DFLT_HIGH`   |                       |

Documentation has been written to explain in more detail the meaning of
the flags and how they can be used.

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