ARM: dts: msm: add reg names dt property for watchdog

Add reg-names property in device tree and modify driver to honor this
property. This is in preparation for using a register to disable watchdog
for one of the clusters on 8994 target.

Change-Id: Ic2d379daa34b9773f263d3ca685bab31a9b001f7
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>