Further rework is going to introduce a dedicated storage for transfer
register value in struct sys_reg_params. Before doing this we have to
remove 'const' modifiers from it in all accessor functions and their
callers.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>