dts/bindings: iis2dlpc: create a common st,iis2dlpc-common.yaml

Create a common binding file that will be included by all bindings
handled by iis2dlpc driver. For now this includes optional drdy-gpios
property and it is shared by the i2c and spi version of the driver.

Signed-off-by: Armando Visconti <armando.visconti@st.com>