lib: utils/serial: Update Shakti UART based on latest implementation

The age old version of Shakti UART was upgraded long back, but we missed
updating the driver in OpenSBI. The old version of UART is not supported
anymore, hence removed the inline comment which is also outdated now.

Signed-off-by: Prasanna T <ptprasanna@gmail.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>