platform/mellanox: fix the mlx-bootctl sysfs

This is a follow-up commit for the sysfs attributes to change
from DRIVER_ATTR to DEVICE_ATTR according to some initial comments.
In such case, it's better to point the sysfs path to the device
itself instead of the driver. The ABI document is also updated.

Fixes: 79e29cb8

 ("platform/mellanox: Add bootctl driver for Mellanox BlueField Soc")
Signed-off-by: Liming Sun <lsun@mellanox.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>