spi: Update the "master/slave" terminology in documentation

Update the master/slave terminology wherever possible to adopt usage of
the controller/host/target. Some parts have been left untouched because
they were sysfs entries and will probably end up being inaccurate if
simply replaced here.

Signed-off-by: Dhruva Gole <d-gole@ti.com>
Link: https://msgid.link/r/20240215085404.1711976-1-d-gole@ti.com


Signed-off-by: Mark Brown <broonie@kernel.org>