dts: bindings: nordic,nrf21540-fem: add supply voltage parameter

This commit adds an optional property to the nRF21540 Front-End Module
devicetree description that specifies supply voltage in mV. This
property can be used by the nRF21540 driver to compensate the value of
achieved gain for different supply voltage.

Signed-off-by: Artur Hadasz <artur.hadasz@nordicsemi.no>
Signed-off-by: Jędrzej Ciupis <jedrzej.ciupis@nordicsemi.no>