Clarify chipselect silkscreen prints

Created by: hzeller

As noted in https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/beagleboard/osw5KrZMRXI/8OxwRwMpAwAJ

.. the chip selects for the SPI bus pins are confusing, as one is CS0, the other is CS1. This clarifies the print on the silkscreen.

Verified that it looks good on KiCAD, but did the same text change also for Eagle.