drivers: pinctrl: gecko: Fix Series 2 I2C location config

Series 2 uses DBUS for alternate function control, which directly uses
port and pin numbers for location selection. Configure pinout as part
of the I2C_SDA/I2C_SCL case, rather than the separate _LOC case,
as the port and pin are not available at this point.

Configure route register prior to enabling the route, to ensure that
the wrong pin is not temporarily claimed.

Fixes an issue where DBUS was always configured to port A and
pin given by the GECKO_LOC() macro, rather than the correct port/pin
pair.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>