hw/arm/armsse.c: Correct modelling of SSE-300 internal SRAMs

hw/arm/armsse.c: Correct modelling of SSE-300 internal SRAMs

The SSE-300 was not correctly modelling its internal SRAMs:
 * the SRAM address width default is 18
 * the SRAM is mapped at 0x2100_0000, not 0x2000_0000 like
   the SSE-200 and IoTKit

The default address width is no longer guest-visible since
our only SSE-300 board sets it explicitly to a non-default
value, but following the hardware's default will help for
any future boards we need to model.

Reported-by: Devaraj Ranganna <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20210510190844[email protected]