docs: split up serial-interfaces.rst

2016-09-06T15:14:52Z

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 Signed-off-by: Jonathan Corbet

kernel/Documentation/driver-api/spi.rst, branch linux-5.1.y

docs: split up serial-interfaces.rst