[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:

https://static.docs.arm.com/epm128372/20/arm_cortex_a55_software_optimization_guide_v2.pdf

Original patch by Javed Absar.

Differential Revision: https://reviews.llvm.org/D46884