platforms/rdn2cfg3: add model startup support for cfg3 variant

Add initial model startup support for RD-N2-Cfg3 platform. This
supports busybox, buildroot, distro boot, secure boot and acs tests.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: Ic93f1eed0659b21133f89020c65e031f572543a5