Remove burst length from DT binding

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Make legacy mode a boolean DT property and remove it as module parameter

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Explain better the legacy mode

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>