sata_rcar: extend PM methods

In order to make it possible to restore from hibernation not only in Linux but
also in e.g. U-Boot, we have to extend  sata_rcar_{suspend|resume}() to {freeze|
thaw}() PM  methods and implement  the  restore() PM method.

Signed-off-by: Mikhail Ulyanov <mikhail.ulyanov@cogentembedded.com>
[Sergei: killed unused variable, changed the order of initializers, modified
copyrights, renamed, modified changelog.]
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tejun Heo <tj@kernel.org>