Factor non-secure system initialisation

When we add PSCI, we'll want to share the same non-secure sysetem
initialisation code. As we're going to want to put spin-table and PSCI
implementations in separate files, it would be nice to have the
initialisation code in its own file, to make clear the separation
between early boot, platform interface code, and non-secure system
initialisation.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>