intel_galileo_rev_g.c: Retrieve GPIO base numbers from device paths

A first step to enable mraa on upstream kernels: use
mraa_get_gpiochip_base to retrieve the base numbers for all GPIOs we use
as output or mux pin. Devices paths are used as stable anchors.

Upstream kernels generate conceptually unpredictable numberings because
they prefer dynamic base numbers over the hard-coded bases the vendor
kernels use so far.

GPIO usage has been tested successfully on an IOT2040 with both Yocto
and upstream kernel.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>