drivers: modem: ublox-sara-r4: Support SARA-U2 modems, sense VINT

This adds support for SARA-U2 modems. They have different timings on
the PWR_ON pin, don't support AT+CESQ and require a manual GPRS
connection setup.

The VINT pin is used as a more reliable and faster way to power on the
modem.

Based on work by Göran Weinholt <goran.weinholt@endian.se>

Signed-off-by: Michael Scott <mike@foundries.io>