Serial: Apply OOP model by MCU_Driver-HAL

For more information see:
* MCU-Driver-HAL programming model: https://gitlab.oss.arm.com/engineering/iot-m-sw/open-iot-sdk/mcu-driver-hal/mcu-driver-hal/-/blob/main/docs/porting/programming_model.md
* Mbed OS migration guide: https://gitlab.oss.arm.com/engineering/iot-m-sw/open-iot-sdk/mcu-driver-hal/mcu-driver-hal/-/blob/main/docs/porting/porting_full_process/migrating_mbed_os_hal.md



Note: `PinMap` usage to be removed once all driver implementations have been migrated.
Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>