i2c: create debugfs entry per adapter

i2c: create debugfs entry per adapter

[ Upstream commit 73febd775bdbdb98c81255ff85773ac410ded5c4 ]

Two drivers already implement custom debugfs handling for their
i2c_adapter and more will come. So, let the core create a debugfs
directory per adapter and pass that to drivers for their debugfs files.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>
Stable-dep-of: 8d3cefaf6592 ("i2c: core: Lock address during client device instantiation")
Signed-off-by: Sasha Levin <sashal@kernel.org>