platform/x86: mlx-platform: Add deffered bus functionality

mlx-platform activates i2c-mux-reg, which creates buses needed by
mlxreg-hotplug. If the mlxreg-hotplug probe runs before the i2c-mux-reg
probe completes, it may attempt to connect a device to an adapter number
that has not been created yet, and fail.

Make mlx-platform driver record the highest bus number in mlxreg-hotplug
platform data and defer mlxreg-hotplug probe until all the buses are
created.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
[dvhart: rewrite commit message more concisely]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>