feat(simd): add data struct for simd ctxt management

From a context management perspective, we refer to SVE and FP state
as SIMD. SVE can be considered as a superset of FP/NEON functionality
and there is a architectural dependency between the two due to the fact
that FP registers occupy the bottom 128 bits of SVE Z vector registers.

This patch introduces a new data structure to hold the FP state of a CPU
as needed for context management operations during world switch events.
It will evolve in next patches to capture the SVE state as well.

Having this unified data structure is necessary for us to implement
SIMD context management while catering to various build configurations.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Signed-off-by: Okash Khawaja <okash@google.com>
Change-Id: If2a9a7526883780df49bbd3dade2f09f7c3d95ae