drivers/sensor: iis2dlpc: use common stmemsc routine

This commit aligns iis2dlpc sensor driver to latest multi
instance sensor driver model.

In particular:

    1. make use of some few DT helpers:
        - get bus devices with DEVICE_DT_GET
        - get SPI information with SPI_CONFIG_DT_INST
        - get drdy gpios with GPIO_DT_SPEC_GET

    2. make use of the stmemsc common routines and move ctx
       handler inside struct config, so that the bus_init
       routines can be totally avoided.

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