pci: mx6: Implement power callback

pci: mx6: Implement power callback

Implement a callback to toggle the slot power supply. The callback
can be overriden in case some more complex power supply for the slot
was implemented in hardware, yet for the usual case, one can define
a GPIO which toggles the power to the slot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Liu Ying <Ying.Liu@freescale.com>