pm8xxx-adc: Fix lockdep warnings with key's not in .data

Devices attributes have lockdep keys and so they should exist in
the data section. Failure to do this results in lockdep turning
itself off and warnings such as:

	BUG: key eeb0883c not in .data!

Fix this problem in the pm8xxx-adc driver by initializing the
keys with sysfs_attr_init().

Change-Id: Ie42f9a28c343e32ef36ca3570d63a7bb557dfe64
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>