ata: ahci: Add sysfs attribute to show remapped NVMe device count

Add a new sysfs attribute to show how many NVMe devices are remapped.

Userspace like distro installer can use this info to ask user to change
the BIOS setting.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>