arm/ptw: Handle atomic updates of page tables entries in MMIO during PTW.

arm/ptw: Handle atomic updates of page tables entries in MMIO during PTW.

I'm far from confident this handling here is correct. Hence
RFC.  In particular not sure on what locks I should hold for this
to be even moderately safe.

The function already appears to be inconsistent in what it returns
as the CONFIG_ATOMIC64 block returns the endian converted 'eventual'
value of the cmpxchg whereas the TCG_OVERSIZED_GUEST case returns
the previous value.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-id: 20240219161229.11776-1-Jonathan.Cameron@huawei.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>