ARM: mach-bcm: disable ftrace in SMC invocation routines

The SMC calling convention uses R7 as an argument register, which
conflicts with its use as a frame pointer when building in Thumb2 mode.
Given that Clang with ftrace does not permit frame pointers to be
disabled, let's omit this compilation unit from ftrace instrumentation.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>