Documentation: devres: add missing I2C helper

Add missing devm_i2c_add_adapter() to devres.rst. It's introduced by
commit 07740c92 ("i2c: core: add managed function for adding i2c
adapters").

Fixes: 07740c92 ("i2c: core: add managed function for adding i2c adapters")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>