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

Add property "qcom,dtest-buffer" to specify which dtest rail to feed
when the pin is configured as a digital input.

Change-Id: I05b253147677ca66d926eaeaa680bd09e31247a5
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>