hw/arm/sbsa-ref: Tidy up use of RAMLIMIT_GB definition

hw/arm/sbsa-ref: Tidy up use of RAMLIMIT_GB definition

Define RAMLIMIT_BYTES using the TiB definition and display
the error parsed with size_to_str():

  $ qemu-system-aarch64-unsigned -M sbsa-ref -m 9T
  qemu-system-aarch64-unsigned: sbsa-ref: cannot model more than 8 TiB of RAM

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250623121845.7214-22-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>