infra/common: Update memory address for soc expansion block

In recent Arm reference design Fixed Virtual Platforms (FVP) the SoC
expansion block is moved to a new memory map from 0xC000_0000_0000 to
0x10_8000_0000. Update the mention of addresses in the doc accordingly.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>