product/rdn2: use platform config to find primary cpu

Primary CPU number is hardcoded at multiple places and so this makes it
difficult to use a different CPU as the primary CPU. So add support to
specify the primary CPU's MPID number via platform config and calculate
logical number during runtime.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: Ifb15f1525de44baabb9038050478b301c9d272b6