[AArch64] This is a work in progress to provide a machine description
for the Cortex-A53 subtarget in the AArch64 backend.

This patch lays the ground work to annotate each AArch64 instruction
(no NEON yet) with a list of SchedReadWrite types. The patch also
provides the Cortex-A53 processor resources, maps those the the default
SchedReadWrites, and provides basic latency. NEON support will be added
in a subsequent patch with proper forwarding logic.

Verification was done by setting the pre-RA scheduler to linearize to
better gauge the effect of the MIScheduler. Even without modeling the
forward logic, the results show a modest improvement for Cortex-A53.

Reviewers: apazos, mcrosier, atrick
Patch by Dave Estes <cestes@codeaurora.org>!