sensor: vl53l0x: make xshut pin control optional

The XSHUT pin, used to shutdown the device when it is not
in current use, is optional and some boards (like ArgonKey)
leave it tied to vdd. So, the driver must provide a way to
make this part configurable.

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