ARM: dts: msm: add sync write delay of WLED for QRD8937

Add DT property for sync write delay for WLED module, which is needed
between two consecutive sync register writes, for MSM8937 QRD that
uses PMI8950 and PMI8937.

Change-Id: I9245437045d471022ac64ec80c8d57b3f7656b55
Signed-off-by: Mao Li <maol@codeaurora.org>