mfd: rtsx: Add shutdown callback in rtsx_pci_driver

Some actions to clear power state should be handled in .shutdown
callback in rtsx_pci_driver. This patch adopts the following measures to
catch this goal:
1. Add a function rtsx_pci_power_off to abstract the common ops in
.shutdown and .suspend
2. Add pcr->ops->force_power_down to fulfill the individual action for
each reader model

Signed-off-by: Wei WANG <wei_wang@realsil.com.cn>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>