boards: shields: ssd1306_128x64_spi: fix GPIO usage

Move the data/command GPIO from Arduino header 16 (D10), which collides
with Arduino SPI SS, to Arduino header 15 (D9).

Add commented example for specifying a reset GPIO on Arduino header 14
(D8).

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>