docs: driver-api: device-io: Document ioremap() variants & access funcs

This documents the newly introduced ioremap_np() along with all the
other common ioremap() variants, and some higher-level abstractions
available.

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