hw/arm/aspeed_ast27x0: Fix RAM size detection failure on BE hosts

hw/arm/aspeed_ast27x0: Fix RAM size detection failure on BE hosts

On big-endian hosts, the aspeed_ram_capacity_write() function previously passed
the address of a 64-bit "data" variable directly to address_space_write(),
assuming host and guest endianness matched.

However, the data is expected to be written in little-endian format to DRAM.
On big-endian hosts, this led to incorrect data being written into DRAM,
which caused the guest firmware to misdetect the DRAM size.

As a result, U-Boot fails to boot and hangs.

- Replaces the "address_space_write()" call with "address_space_stl_le()",
  which performs an explicit 32-bit little-endian write.
- Updating the MemoryRegionOps to restrict access to exactly 4 bytes
  using .valid.{min,max}_access_size = 4 and .impl.min_access_size = 4.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Fixes: 7436db1 ("aspeed/soc: fix incorrect dram size for AST2700")
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250522023305.2486536-4-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>