xen/arm: vreg: Introduce vreg_emulate_cp{32,64}

xen/arm: vreg: Introduce vreg_emulate_cp{32,64}

Factorize the code to emulate 32-bit and 64-bit access to a co-processor
in specific helpers.

The new helpers will be used in different components to simplify the
emulation.

Finally, the prototypes for the callbacks to emulate 32-bit and 64-bit
co-processor access are the same as the sysreg one. Rather than
introducing new ones, repurpose the existent prototypes.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>