leds: leds-qpnp-wled: add delay between consecutive writes to WLED SYNC

For PMi8950 WLED, consecutive writes to SYNC register of WLED SINK block
don't take effect unless a delay is introduced between the two writes.
Add an optional DT property which indicates the duration of this delay
and corresponding driver changes for the same.

Change-Id: I04dea7772233e38ce47854e1a4cc55e633b0f2be
Signed-off-by: Abinaya P <abinayap@codeaurora.org>