lib/string_helpers: Add str_read_write() helper

Add str_read_write() helper to return 'read' or 'write' string literal.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dmitry Rokosov <ddrokosov@sberdevices.ru>
Link: https://lore.kernel.org/r/20220822175011.2886-2-ddrokosov@sberdevices.ru


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>