It's no use talking about racism and then continue using
terms like master and slave. The X400 SPI interface is still
pretty new, so without interfering with too much legacy we
should still be able to jump onto a new, better naming scheme
for the SPI signals. Following the Open Source Hardware
Association's Resolution to Redefine SPI Signal Names
<https://www.oshwa.org/a-resolution-to-redefine-spi-signal-names/>
we will use Controller and Peripheral, Serial Data Out
(SDO) and Serial Data In (SDI) and Chip Select (CS).
Leftovers like MISO and MOSI are legacy from long existing
APIs which cannot be changed easily without breaking user
applications.
Add support for reading the number of supported SPI slaves from
the device. This has become necessary because we may have bitfiles
with different capabilities and we want to report this back correctly.
