hw/char: cadence_uart: Log a guest error when device is unclocked or in reset

hw/char: cadence_uart: Log a guest error when device is unclocked or in reset

We've got SW that expects FSBL (Bootlooader) to setup clocks and
resets. It's quite common that users run that SW on QEMU without
FSBL (FSBL typically requires the Xilinx tools installed). That's
fine, since users can stil use -device loader to enable clocks etc.

To help folks understand what's going, a log (guest-error) message
would be helpful here. In particular with the serial port since
things will go very quiet if they get things wrong.

Suggested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210901124521.30599-7-bmeng.cn@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>