edtlib: allow register addr / size to be None

If the #address-cells property for a register is 0 than we set the addr
value of the reg to None.  Similar, if #size-cells is 0 than we set the
size value to None for the reg.

Fixup kconfigfunctions.py to handle reg.size and reg.addr being None.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>