arm_fpga: Add platform documentation

As the Arm Ltd. FPGA port is now working for all existing images, add
some documentation file.

Change-Id: I9e2c532ed15bbc121bb54b3dfc1bdfee8f1443a6
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/docs/plat/arm/index.rst b/docs/plat/arm/index.rst
index 1afe475..9c2fcb1 100644
--- a/docs/plat/arm/index.rst
+++ b/docs/plat/arm/index.rst
@@ -9,6 +9,7 @@
    fvp/index
    fvp-ve/index
    tc0/index
+   arm_fpga/index
    arm-build-options
 
 This chapter holds documentation related to Arm's development platforms,