mlxbf-bootctl: Add sysfs file for BlueField boot fifo

mlxbf-bootctl: Add sysfs file for BlueField boot fifo

This commit adds sysfs file for BlueField boot fifo. The boot
fifo is usually used to push boot stream via USB or PCIe. Once
OS is up, it can be reused by applications to read data or
configuration from external host.

Signed-off-by: Liming Sun <limings@nvidia.com>
Reviewed-by: David Thompson <davthompson@nvidia.com>
Link: https://lore.kernel.org/r/52b0b00dacbc4aad3169dd3667d79c85e334783b.1680657571.git.limings@nvidia.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>