hw/misc/aspeed_scu: Set MemoryRegionOps::impl::access_size to 32-bit

hw/misc/aspeed_scu: Set MemoryRegionOps::impl::access_size to 32-bit

All MemoryRegionOps::read/write() handlers switch over a 32-bit
aligned value, because converted using TO_REG(), which is defined
as:

  #define TO_REG(offset) ((offset) >> 2)

So all implementations are 32-bit.
Set min/max access_size accordingly.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Link: https://lore.kernel.org/qemu-devel/20250331230444.88295-2-philmd@linaro.org
Signed-off-by: Cédric Le Goater <clg@redhat.com>