xen: arm: introduce assembly helper to call smc

xen: arm: introduce assembly helper to call smc

Commit 063188f4b3 "xen: arm: Add support for the Exynos secure firmware"
introduced code assuming that exynos_smc() would get called with arguments in
certain registers. While the "noinline" attribute guarantees the function to
not get inlined, it does not guarantee that all arguments arrive in the assumed
registers: gcc's interprocedural analysis can result in clone functions to be
created where some of the incoming arguments (commonly when they have constant
values) get replaced by putting in place the respective values inside the
clone.

Xen contains in multiple place of this SMC function: consolidate the function
in a single place and write it in assembly.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- reworded commit message as discussed ]