arm/dt: Update AMUX channel number property

Update AMUX devicetree channel node to include the reg property.

Each channel node has an index value to represent the AMUX channel
number that is used to select the corresponding analog input. The
reg property can be used to represent the above channel number
and conforms with the devicetree spec.

Change-Id: Ica53255354006ca44c57b6ca13e84fd7fb6b02de
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>