docs: driver-api: device-io: Document I/O access functions

This adds more detailed descriptions of the various read/write
primitives available for use with I/O memory/ports.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hector Martin <marcan@marcan.st>