boards: silabs: Remove usage of no-op pin number macro

The GECKO_PIN() macro does not do anything except pass the
argument through unaltered. It only serves to make DeviceTree
files more verbose. It was inconsistently used, make the .dts
files consistent by never using it.

The DBUS pinctrl driver doesn't use the port or location macros
from the gpio_gecko.h header. The pin number macro is the only
other thing defined in this header. Stop including the header on
Series 2 based boards.

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