sata_via: Enable optional hotplug on VT6420

VT6420 seems to have the same hotplug capability as VT6421.

However, enabling hotplug needs to expose SCR registers which can cause
problems. It works for me but might break elsewhere. So add a module
parameter vt6420_hotplug to enable this feature.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Tejun Heo <tj@kernel.org>