want UART methods to be virtual methods.

[tokiedokie]

MbedSPI and MbedI2C methods are declared as virtual methods. However the UART methods are not.

Since we want to create our own driver by inheriting from UART, it would be nice if the UART methods were also declared as virtual methods.

https://github.com/arduino/ArduinoCore-mbed/blob/1399d6477548f3f5585155652511d68d3ef85951/libraries/Wire/Wire.h#L34-L68

https://github.com/arduino/ArduinoCore-mbed/blob/1399d6477548f3f5585155652511d68d3ef85951/libraries/SPI/SPI.h#L28-L48

https://github.com/arduino/ArduinoCore-mbed/blob/1399d6477548f3f5585155652511d68d3ef85951/cores/arduino/Serial.h#L39-L67