hw/char/riscv_htif: Clarify MemoryRegionOps expect 32-bit accesses

hw/char/riscv_htif: Clarify MemoryRegionOps expect 32-bit accesses

Looking at htif_mm_ops[] read/write handlers, we notice they
expect 32-bit values to accumulate into to the 'fromhost' and
'tohost' 64-bit variables. Explicit by setting the .impl
min/max fields.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20241129154304.34946-4-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>