dtc: Warning rather than error on possible truncation of cell values

We always evaluate integer values in cell arrays as 64-bit quantities, then
truncate to the size of the array cells (32-bit by default).  However to
detect accidental truncation of meaningful values, we give an error if the
truncated portion isn't either all 0 or all 1 bits.  However, this can
still give counterintuitive errors.  For if the user is thinking in 2's
complement 32-bit arithmetic (which would be quite natural), then they'd
expect the expression (-0xffffffff-2) to evaluate to -1 (0xffffffff).
However in 64-bit it evaluates to 0xfffffffeffffffff which does truncate
to the expected value but trips this error message.

Because of this reduce the error to only a warnings, with a somewhat more
helpful message.

Fixes: https://github.com/dgibson/dtc/issues/74

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/cell-overflow-results.dts

@@ -0,0 +1,7 @@
+/dts-v1/;
+
+/ {
+	prop1 = < 0 >;
+	prop2 = < 0xffffffff >;
+	prop3 = < 0 >;
+};