PCI/switchtec: Add Gen4 flash information interface support

Add the new flash_info registers struct and the implementation of
ioctl_flash_part_info() for the new Gen4 hardware.

[logang@deltatee.com: rewrote commit message]
Link: https://lore.kernel.org/r/20200115035648.2578-7-logang@deltatee.com


Signed-off-by: Kelvin Cao <kelvin.cao@microchip.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>