spi: add binding for clps711x SPI

This documents the binding used by Alexander Shiyan's DT support for
the clps711x SPI controller.

I've left the file name to match the ARM platform port name "clps711x"
for consistency with the other bindings, even though the compatible
string refers to the later ep7309 chip.

Linux no longer supports the old clps711x and ep72xx product lines,
but we still use the name. The entire family is now discontinued
by the manufacturer.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>