RSE: Add getter function for RSE ID

Add a function declaration for a function to get the RSE ID and a
default implementation which fetches the value from the OTP. Platforms
can define their own version of this function in the case where the RSE
ID is not in the OTP.

Change-Id: I8d932aa82dbe00bb52376ac398ab8d6284fec0fd
Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>
7 files changed