Documentation: fpga: dfl: add link address of feature id table

This patch adds the link address of feature id table in documentation.

Signed-off-by: Tianfei zhang <[e-mail]>
Reviewed-by: Matthew Gerlach <[e-mail]>
Acked-by: Moritz Fischer <[e-mail]>
Acked-by: Wu Hao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Xu Yilun <[e-mail]>

Documentation/fpga/dfl.rst

···
 FME Partial Reconfiguration Sub Feature driver (see drivers/fpga/dfl-fme-pr.c)
 could be a reference.
 
+Please refer to below link to existing feature id table and guide for new feature
+ids application.
+https://github.com/OPAE/dfl-feature-id
+
+
 Location of DFLs on a PCI Device
 ================================
 The original method for finding a DFL on a PCI device assumed the start of the