Hi,
Sorry about that, this is a bug of the new mbed BSP that is in the Beta stage.
To fix this, please copy the %LOCALAPPDATA%\VisualGDB\EmbeddedBSPs\arm-eabi\com.sysprogs.arm.stm32\STM32F4xxxx\DeviceDefinitions\stm32f446xx.xml.gz file to %LOCALAPPDATA%\VisualGDB\EmbeddedBSPs\arm-eabi\com.sysprogs.arm.mbed\DeviceDefinitions\stm32f446xx.xml.gz and add the following line to the mbed.xml file in your project directory under the EmbeddedProfile element:
<MCUDefinitionFile>DeviceDefinitions/stm32f446xx.xml</MCUDefinitionFile>
This will automatically pick the register definitions from the normal STM32 package.