Documentation: devres: add missing entry for devm_platform_ioremap_resource()

devm_platform_ioremap_resource() should be documented in devres.rst.
Add the missing entry.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20191022084318.22256-2-brgl@bgdev.pl
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>