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This patch defines a SMCC context to save and restore
registers during a SMC call. It also adds appropriate helpers
to save and restore from this context for use by AArch32
secure payload and BL stages.

Change-Id: I64c8d6fe1d6cac22e1f1f39ea1b54ee1b1b72248

This patch adds the essential AArch32 architecture helpers
arch.h and arch_helpers.h and modifies `_types.h` to add AArch32
support.

A new build option `ARCH` is defined in the top level makefile to
enable the component makefiles to choose the right files based on the
Architecture it is being build for. Depending on this flag, either
`AARCH32` or `AARCH64` flag is defined by the Makefile. The default
value of `ARCH` flag is `aarch64`. The AArch32 build support will be
added in a later patch.

Change-Id: I405e5fac02db828a55cd25989b572b64cb005241