backlight: qcom-spmi-wled: Add support to configure slew ramp time

Add support to configure slew ramp time for brightness change for
PM7325B by adding a DT property named "qcom,slew-ramp-time".
Also set slew rate to lowest value when enabling or disabling WLED
to ensure fast turning on/off of display and restore it to the
specified value when display is turned on.

Change-Id: I63ae34702b934f45ffab39d521a6ecc0a6c8ccd5
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>