platforms/rdfremontcfg2: update rss2rss mhu channel count

The current driver in RSS for MHUv2 requires multiple channels for
cross-chip RSS to RSS communication. However, the RD_Fremont_Cfg2 FVP
only supports one channel for RSS to RSS MHUs. Therefore, we need to
increase the channel count to accommodate the driver's requirement for
multiple channels.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I0e6f40d754783a9bbea9fc267e199d75a1beb867