ARM: dts: msm: Use phandles for PM8941 local slave IDs

Add a pair of phandles to represent PM8941 local slave IDs 0 and
1.  The global slave ID used in SPMI transactions corresponding
to these local slave IDs changes depending upon whether or not
PM8941 is the primary PMIC in the system.

Using phandles for the local slave IDs allows the global slave
IDs for the local slave IDs to be specified on a per board basis.
This is required for MSM8974Pro-AC where the global slave IDs for
PM8941 change.

Change-Id: Idfe8a73467d5cfab2b206706df969f12b277b4ce
Signed-off-by: David Collins <collinsd@codeaurora.org>