platform/x86: mlx-platform: Use defines for bus assignment

Add defines for the bus IDs, used for hotplug device topology to improve
code readability. Defines added for FAN and power units.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>