ttwrplus: add battery voltage readout

Add battery voltage readout through the XPowersLib library.

TG-553

platform/targets/ttwrplus/platform.c

@@ -82,7 +82,7 @@ void platform_terminate()
 
 uint16_t platform_getVbat()
 {
-    return 0;
+    return pmu_getVbat();
 }
 
 uint8_t platform_getMicLevel()

platform/targets/ttwrplus/pmu.cpp

@@ -292,3 +292,8 @@ void pmu_init()
     val = PMU.getChargeTargetVoltage();
     printk("Setting Charge Target Voltage : %d\n", tableVoltage[val]);
 }
+
+uint16_t pmu_getVbat()
+{
+    return PMU.isBatteryConnect() ? PMU.getBattVoltage() : 0;
+}

platform/targets/ttwrplus/pmu.h

@@ -25,7 +25,7 @@ extern "C" {
 #endif
 
 void pmu_init();
-
+uint16_t pmu_getVbat();
 
 #ifdef __cplusplus
 }