ata: sata_rcar: exclude setting of PHY registers in Gen3

According to documentation, setting of PHY registers is unnecessary with
R-Car Gen3. The registers are not even described. So, don't initialize
them.

Signed-off-by: Masaharu Hayakawa <masaharu.hayakawa.ry@renesas.com>
[wsa: updated commit message]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tejun Heo <tj@kernel.org>