Platform/Sgi: update fastchannel address for RD-N2 variants

For RD-N2 and variant platforms, the shared SRAM memory carveouts have
changed due to updates in the platform firmware. Correspondingly,
update the fastchannel address to align with the rest of the platform
firmware components.

Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Change-Id: I5a2477c58b0ee58231915886a9a2937c5f7f6ba9