arm: fconf: Fix GICv3 dynamic configuration

At the moment the fconf_populate_gicv3_config() implementation is
somewhat incomplete: First it actually fails to store the retrieved
information (the local addr[] array is going nowhere), but also it makes
quite some assumptions about the device tree passed to it: it needs to
use two address-cells and two size-cells, and also requires all five
register regions to be specified, where actually only the first two
are mandatory according to the binding (and needed by our code).

Fix this by introducing a proper generic function to retrieve "reg"
property information from a DT node:
We retrieve the #address-cells and #size-cells properties from the
parent node, then use those to extract the right values from the "reg"
property. The function takes an index to select one region of a reg
property.

This is loosely based on the STM32 implementation using "reg-names",
which we will subsume in a follow-up patch.

Change-Id: Ia59bfdf80aea4e36876c7b6ed4d153e303f482e8
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/arm/board/fvp/jmptbl.i b/plat/arm/board/fvp/jmptbl.i
index 6469e29..50a5ba4 100644
--- a/plat/arm/board/fvp/jmptbl.i
+++ b/plat/arm/board/fvp/jmptbl.i
@@ -25,6 +25,9 @@
 fdt     fdt_next_subnode
 fdt     fdt_path_offset
 fdt     fdt_subnode_offset
+fdt     fdt_address_cells
+fdt     fdt_size_cells
+fdt     fdt_parent_offset
 mbedtls mbedtls_asn1_get_alg
 mbedtls mbedtls_asn1_get_alg_null
 mbedtls mbedtls_asn1_get_bitstring_null