[AArch64] Cortex-A55 scheduler model
This is an initial commit adding the A55 model, but it isn't used/enabled yet.
We will follow up on this to improve the model, then flip the switch.
The optimisation guide describing Cortex-A55 micro-architecture in more detail
can be found here:
Original patch by Javed Absar.
Differential Revision: https://reviews.llvm.org/D46884