neoverse-rd/rdv3: rearrange scp_platform module context structure

The SCP platform module is implemented across multiple C source files.
To ensure the module's context data is accessible to all source files,
move the context structure definition from the source file to the
header file.Move scp_platform module's context structure from the
source file to the header file.

Change-Id: I2ffeb382482bcadb25b17352501a7a8f95ae78b6
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>