fpga: altera-cvp: Discover Vendor Specific offset

Newer Intel FPGAs have different Vendor Specific offsets than
legacy parts. Use PCI discovery to find the CvP registers.
Since the register positions remain the same, change the hard
coded address to a more flexible way of indexing registers
from the offset.
Adding new PCI read and write abstraction functions to
handle the offset (altera_read_config_dword() and
altera_write_config_dword()).

Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>