ARM: dts: msm: increase the external buck settling time

Increase the external buck settling time from 133us
to 500us. This increases the system stability.

CRs-Fixed: 621198
Change-Id: I4aed65c007f8d377646d5c828b09c6f88c213928
Signed-off-by: Anil kumar mamidala <amami@codeaurora.org>