target-arm: Add raw_readfn and raw_writefn to ARMCPRegInfo
    
For reading and writing register values from the kernel for KVM,
we need to provide accessor functions which are guaranteed to succeed
and don't impose access checks, mask out unwritable bits, etc.
Define new fields raw_readfn and raw_writefn for this purpose;
these only need to be provided if there is a readfn or writefn
already and it is not suitable.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
