hw/misc/bcm2835_rng: Specify valid memory access sizes

The BCM2835 RNG has 32-bit registers only; specify this in
the MemoryRegionOps so wrong-sized accesses are rejected rather
than getting to the assertions in the read and write functions,
and for clarity add the matching .impl constraints.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3394
Fixes: 54a5ba13a9f ("target-arm: Implement BCM2835 hardware RNG")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20260501162700.4092512-1-peter.maydell@linaro.org