soc: pinctrl-lpi: add changes to support slew rate config

Add changes to read slew rate from device tree table
and set the same under set configs list.

Change-Id: I0d7e950d67d34b63c5a6436bb4d10b08a80c2c58
Signed-off-by: Karthikeyan Mani <kmani@codeaurora.org>