hw/char/bcm2835_aux: Allow less than 32-bit accesses

hw/char/bcm2835_aux: Allow less than 32-bit accesses

The "BCM2835 ARM Peripherals" datasheet [*] chapter 2
("Auxiliaries: UART1 & SPI1, SPI2"), list the register
sizes as 3/8/16/32 bits. We assume this means this
peripheral allows 8-bit accesses.

This was not an issue until commit 5d971f9e67 which reverted
("memory: accept mismatching sizes in memory_region_access_valid").

The model is implemented as 32-bit accesses (see commit 97398d900c,
all registers are 32-bit) so replace MemoryRegionOps.valid as
MemoryRegionOps.impl, and re-introduce MemoryRegionOps.valid
with a 8/32-bit range.

[*] https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf

Fixes: 97398d900c ("bcm2835_aux: add emulation of BCM2835 AUX (aka UART1) block")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201002181032.1899463-1-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>