adc: current_sense_amplifier: resistance in milli-ohms

Change the unit of the sense resistor in the devicetree binding from
micro-ohms to milli-ohms. This is done for three reasons.

Firstly, the maximum value resistor that can currently be represented
is 4.2 kOhms, due to the limitation of devicetree properties to 32 bits.

Secondly, storing the resistance at such a high resolution makes
overflows much more likely when the desired output unit is micro-amps,
not milli-amps.

Finally, micro-ohms, are an unnecessarily precise unit for the purpose
of these calculations, and a resolution that is not realistic to
achieve. The high resistor resolution results in large divisors that
reduce the resolution of outputs. Unlike resistors characterised down to
the micro-ohm, devices wanting to measure micro-amps are actually
realistic.

Signed-off-by: Jordan Yates <jordan@embeint.com>