feat(el3-runtime): introduce system register trap handler

At the moment we only handle SMC traps from lower ELs, but ignore any
other synchronous traps and just panic.
To cope with system register traps, which we might need to emulate,
introduce a C function to handle those traps, and wire that up in the
exception handler to be called.

We provide a dispatcher function (in C), that will call platform
specific implementation for certain (classes of) system registers.
For now this is empty.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: If147bcb49472eb02791498700300926afbcf75ff