mfd: ab8500: Drop debugfs module

This debugfs isn't used by anyone, if we want to dump the contents
of registers we should just convert the driver to regmap instead.
I'm also excluding this from the device tree bindings when converting
to YAML because it is not a real device, and device trees should
only contain real physical devices.

Delete it.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220212214724.681530-1-linus.walleij@linaro.org