Add VPM size to detectPlatform output

Now that DMA is enabled, it's interesting to know what the actual size is of the VPM.
This adds it to the output of `detectPlatform`.

Example output:

```
> sudo obj-qpu/bin/detectPlatform
Detected platform: Raspberry Pi 2 Model B Rev 1.1
Hardware revision: a01041
Number of slices: 3
Number of QPU's per slice: 4
Size of VPM: 12KB                    # <-- This is new

```

**NOTE:* This will probably not work on your machine until mn416#52 and mn416#53 have been merged.

Lib/VideoCore/RegisterMap.cpp

@@ -72,6 +72,20 @@ int RegisterMap::numQPUPerSlice() {
 }
 
 
+/**
+ * @brief get the size of the VPM.
+ *
+ * @return Size of VPM in KB
+ */
+int RegisterMap::VPMSize() {
+	uint32_t reg = readRegister(V3D_IDENT1);
+	int value = (reg >> 28) & 0xf;
+
+	if (value == 0) return 16;  // According to reference doc p.97
+	return value;
+}
+
+
 RegisterMap *RegisterMap::instance() {
 	if (m_instance.get() == nullptr) {
 		m_instance.reset(new RegisterMap);

Lib/VideoCore/RegisterMap.h

@@ -26,6 +26,7 @@ class RegisterMap {
 
 	static int numSlices();
 	static int numQPUPerSlice();
+	static int VPMSize();
 
 private:
 	RegisterMap();

Tools/detectPlatform.cpp

@@ -60,6 +60,7 @@ int main(int argc, char *argv[]) {
 	if (geteuid() == 0) {  // Only do this as root (sudo)
 		printf("Number of slices: %d\n", RegisterMap::numSlices());
 		printf("Number of QPU's per slice: %d\n", RegisterMap::numQPUPerSlice());
+		printf("Size of VPM: %dKB\n", RegisterMap::VPMSize());
 	} else {
 		printf("You can see more if you use sudo\n");
   }