Add support for Raspberry Pi 4B

This change make the following commands correctly detect the 4B
however there may be an issue with reading GPIO inputs on the 4B
as they are always returning zero.

The aim of this change is to be the source-equivalent of the binary
package release 2.52[1] which was the last release before upstream
development ceased.

[1] http://wiringpi.com/wiringpi-updated-to-2-52-for-the-raspberry-pi-4b/

gpio/readall.c

@@ -309,6 +309,8 @@ static void plus2header (int model)
     printf (" +-----+-----+---------+------+---+---Pi 3B+-+---+------+---------+-----+-----+\n") ;
   else if (model == PI_MODEL_3AP)
     printf (" +-----+-----+---------+------+---+---Pi 3A+-+---+------+---------+-----+-----+\n") ;
+  else if (model == PI_MODEL_4B)
+    printf (" +-----+-----+---------+------+---+---Pi 4B--+---+------+---------+-----+-----+\n") ;
   else
     printf (" +-----+-----+---------+------+---+---Pi ?---+---+------+---------+-----+-----+\n") ;
 }
@@ -356,6 +358,7 @@ void doReadall (void)
 	(model == PI_MODEL_2)    ||
 	(model == PI_MODEL_3AP)  ||
 	(model == PI_MODEL_3B)   || (model == PI_MODEL_3BP) ||
+	(model == PI_MODEL_4B)   ||
 	(model == PI_MODEL_ZERO) || (model == PI_MODEL_ZERO_W))
     piPlusReadall (model) ;
   else if ((model == PI_MODEL_CM) || (model == PI_MODEL_CM3) || ((model == PI_MODEL_CM3P)))

wiringPi/wiringPi.c

@@ -237,7 +237,7 @@ const char *piModelNames [20] =
   "Pi 3A+",	// 14
   "Unknown15",	// 15
   "CM3+",	// 16
-  "Unknown17",	// 17
+  "Pi 4B",	// 17
   "Unknown18",	// 18
   "Unknown19",	// 19
 } ;

wiringPi/wiringPi.h

@@ -102,6 +102,7 @@
 #define	PI_MODEL_3BP 		13
 #define	PI_MODEL_3AP 		14
 #define	PI_MODEL_CM3P 		16
+#define	PI_MODEL_4B 		17
 
 #define	PI_VERSION_1		0
 #define	PI_VERSION_1_1		1