xen/arm: vgic: Clean-up the sysreg emulation

xen/arm: vgic: Clean-up the sysreg emulation

Couple of clean-up for the vgic sysreg emulation:
    - Reference the public documentation rather than a non-public one
    - Let the vgic emulation decides whether a register needs to be
    emulated
    - Drop unnecessary debug printk. They don't bring much information
    and can be misleading (vGICv2 does not support those registers)

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