boards: nrf: Fix nrf52_pca20020 board for benchmark test.

Currently tests/benchmark/footprint build is failing for
nrf52_pca20020 board. In minimal configuration, all modules are
disabled, including GPIO, because of that defines related to GPIO
are missing in board.c file and compilation fails.

To solve it, I selected GPIO to be always turned on for
nrf52_pca20020 board.

Signed-off-by: Mieszko Mierunski <mieszko.mierunski@nordicsemi.no>