platforms/rdv3cfg2: enable extended spi mode for all chips

With the latest model change (in model version 11.26), model expects the
extended SPI model to be enabled explicitly. Since the software
configures the extended SPI range while configuring for the GIC
mulitchip, enable extended SPI mode by setting extended-spi-count to
960. The value 960 is used as extended SPI count because, chip 2 and
chip 3 allocated 480 extended SPIs each. Since the system treats all the
GIC as single logical entity, the extended-spi-count is set for chip 0
and chip 1 as well.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: Ic9555d493e36510cb0b64240e3be444af17d67fc