MLK-10058-4 pci: imx6: refine imx6sx pcie pm

- Regarding to the pcie design on imx6sx, some gpc
operations are mandatory pre-required when pcie phy
is powered on/off.
In order to DO NOT touch gpc module in pcie driver,
register one pcie phy regulator into gpc, encapsulate
the pcie phy power on/off pre gpc related operations into
regulator's notify and contained in gpc driver
- in order to save power consumption, disable pcie clks and phy
regulator if the pcie link is down.
- remove the PRST set/unset in suspend/resume, because that
usb hub would be reset, and the name of the dev node of the
thumb disk inserted in the port of the pcie2usb device,
would be changed randomly after suspend resume on imx6sx.
- add the extremely power save mode

Signed-off-by: Richard Zhu <Richard.Zhu@freescale.com>