misc: cs40l25: Add plwe ramp down interface

Add a sysfs interface to set the ramp down time on
pwle waveforms. The rampdown time is in steps of 1ms
with a maximum ramp down time of 50ms.  A zero value
indicates that the feature is off.

Bug: 200713702
Test: verified that new node is added and working

Change-Id: I5a8898bd873c47c4d306d0dc1ccc79b082119459
Signed-off-by: Paul Handrigan <Paul.Handrigan@cirrus.com>
Signed-off-by: Vince Leung <leungv@google.com>