convert-dtsv0: Fix signedness comparisons warning

With -Wsign-compare, compilers warn about a mismatching signedness in
comparisons in the generated lexer code.

In this case we walk over an array, and never use negative indicies, so
we can change the loop counter variable to be unsigned.

This fixes "make convert-dtsv0", when compiled with -Wsign-compare.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201012161948.23994-3-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

convert-dtsv0-lexer.l

@@ -94,7 +94,7 @@ static const struct {
 <INITIAL>[0-9a-fA-F]+	{
 			unsigned long long val;
 			int obase = 16, width = 0;
-			int i;
+			unsigned int i;
 
 			val = strtoull(yytext, NULL, cbase);