drivers: gpio: silabs: gpio driver for EFR series 2 devices

Added the gpio driver for EFR series 2 devices.

The SILABS_SISDK_GPIO symbol is added to enable
support for the new GPIO driver.
The SOC_GECKO_GPIO symbol is retained for now to
maintain compatibility with existing drivers and
will be removed in a subsequent commit.

Signed-off-by: S Mohamed Fiaz <fiaz.mohamed@silabs.com>