ata: ahci_platform: enable to get and control reset

Unlike SoC-specific driver, generic ahci_platform driver doesn't
have any chances to control resets.

This adds AHCI_PLATFORM_GET_RESETS to ahci_platform_get_resources()
on the generic driver to enable reset control support.

Suggested-by: Hans de Goede <hdegoede@redhat.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>