spi: Update reference to struct spi_controller

struct spi_master has been renamed to struct spi_controller. Update the
reference in spi.rst to make it clickable again.

Fixes: 8caab75fd2c2 ("spi: Generalize SPI "master" to "controller"")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20221101173252.1069294-1-j.neuschaefer@gmx.net
Signed-off-by: Mark Brown <broonie@kernel.org>

docs: split up serial-interfaces.rst

It never made sense to keep these documents together; move each into its
own file.

Drop the section numbering on hsi.txt on its way to its own file.

Suggested-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>