asoc: bolero: get slew rate value from device tree

trinket has a different slew rate value than talos/moorea.
Hence the slew rate value is read from device tree instead of
using fixed value.

Change-Id: I45955ff91a210e6fe93c70d801673480b7ca978e
Signed-off-by: Ramprasad Katkam <katkam@codeaurora.org>