regmap: fix documentation to match code

The regmap binding talks about one thing, which is register
endianess, and it gets almost every aspect of it wrong.

This replaces the current text of the file with a version
that makes more sense and that matches what we implement
now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: a06c488d ("regmap: Add explict native endian flag to DT bindings")
Fixes: 275876e2 ("regmap: Add the DT binding documentation for endianness")
Signed-off-by: Mark Brown <broonie@kernel.org>