i2c: add core-managed per-client directory in debugfs

More and more I2C client drivers use debugfs entries and currently they
need to manage a subdir for their files on their own. This means
inconsistent naming for these subdirs and they are scattered all over
the debugfs-tree as well. Not to mention the duplicated code.

Let the I2C core provide and maintain a proper directory per client.

Note: It was considered to save the additional pointer in 'struct
i2c_client' and only provide a subdir when requested via a helper
function. When sketching this approach, more and more corner cases
appeared, though, so the current solution with its simple and unabiguous
code was chosen.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>