pinctrl: qcom: spmi-mpp: Add dtest route for digital input

Currently, the driver selects dtest rail specified in "qcom,dtest"
property as the output source in digital output mode. Add another
property "qcom,dtest-buffer" to specify which dtest rail to feed
when the pin is configured as a digital input.

Change-Id: I9e1ae5a64d633c7cdc6a1eb710932ca19ef5fe85
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>