sanitycheck: Improve handling of duplicate devices

Some devices have multiple serial ports, which show as duplicates in the
hardware map. This doesn't work well when using --generate-hardware-map
to regenerate an existing map. E.g. nrf5340pdk_nrf5340_cpuapp by default
prints to the 3rd of 3 devices. If it shows up on /dev/ttyACM0,
/dev/ttyACM1, and /dev/ttyACM2, printout will come on /dev/ttyACM2.
nrf9160dk_nrf9160 also provides 3 devices, but prints to the 1st.

This patch sorts the devices by the serial port and matches multiple
duplicate entries one-to-one to retain the ordering. This way, the
correct device can be given the platform name and the others can be kept
as "unknown" so that --device-testing understands correctly
1) that there is only one device (not 3)
2) where the serial output will come.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>