ARM: bcm: don't special-case CPU 0 in bcm_kona_smc()

There's logic in bcm_kona_smc() to ensure __bcm_kona_smc() gets
called on CPU 0; if already executing on CPU 0, that function is
called directly.  The direct call is not protected from interrupts,
however, which is not safe.

Note that smp_call_function_single() is designed to handle the case
where the target cpu is the current one.  It also gets a reference
to the CPU and disables IRQs across the call.

So we can simplify things and at the same time be protected against
interrupts by calling smp_call_function_single() unconditionally.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>