regulator: cpr-regulator: add a linked list for cpr-regulator devices

Add a global linked list named cpr_regulator_list to keep track
of all cpr-regulator devices.  It can be used when analyzing RAM
dumps in order find out the state of cpr-regulator devices at the
time of a crash.

Change-Id: Ic2abc76c02799edd641e49ee0dfe003614802b11
Signed-off-by: David Collins <collinsd@codeaurora.org>