added raspberry pi 4B benchmark (#1758)

benchmark/README.md

@@ -445,6 +445,40 @@ powersave = 0
   mobilenet-yolo  min =  638.73  max =  641.27  avg =  639.87
 
 ```
+Raspberry Pi 4 Model B Broadcom BCM2711B0, Cortex-A72 (ARMv8) (1.5GHz x 4 )
+```
+pi@raspberrypi:~ $ ./benchncnn 8 4 0
+loop_count = 8
+num_threads = 4
+powersave = 0
+gpu_device = -1
+cooling_down = 1
+          squeezenet  min =   57.47  max =   59.73  avg =   58.73
+     squeezenet_int8  min =   77.41  max =   80.01  avg =   78.72
+           mobilenet  min =   85.06  max =   86.67  avg =   86.01
+      mobilenet_int8  min =  163.69  max =  185.67  avg =  168.48
+        mobilenet_v2  min =   74.13  max =   76.76  avg =   75.84
+        mobilenet_v3  min =   60.93  max =   61.46  avg =   61.25
+          shufflenet  min =   37.62  max =   38.46  avg =   37.98
+       shufflenet_v2  min =   33.00  max =   34.21  avg =   33.38
+             mnasnet  min =   64.39  max =   65.01  avg =   64.64
+     proxylessnasnet  min =   65.71  max =   66.71  avg =   66.06
+           googlenet  min =  175.82  max =  176.69  avg =  176.24
+      googlenet_int8  min =  187.11  max =  188.97  avg =  187.99
+            resnet18  min =  233.36  max =  234.39  avg =  233.89
+       resnet18_int8  min =  156.72  max =  173.10  avg =  159.56
+             alexnet  min =  180.48  max =  197.66  avg =  183.05
+               vgg16  min =  969.88  max = 1007.31  avg =  988.65
+          vgg16_int8  min = 1206.02  max = 1258.90  avg = 1226.27
+            resnet50  min =  480.30  max =  502.61  avg =  486.97
+       resnet50_int8  min =  412.35  max =  465.48  avg =  421.58
+      squeezenet_ssd  min =  183.15  max =  221.97  avg =  190.40
+ squeezenet_ssd_int8  min =  233.73  max =  250.71  avg =  238.20
+       mobilenet_ssd  min =  176.45  max =  197.79  avg =  180.52
+  mobilenet_ssd_int8  min =  259.11  max =  272.16  avg =  261.29
+      mobilenet_yolo  min =  423.25  max =  447.12  avg =  435.26
+  mobilenetv2_yolov3  min =  241.08  max =  283.54  avg =  248.69
+```
 
 Rockchip RK3399 (Cortex-A72 1.8GHz x 2 + Cortex-A53 1.5GHz x 4)
 ```