ARM: bcm: Make secure API call optional

The current bcm_kona_smc_init function throws a BUG_ON if there's no SMC
device node defined in the device tree.

Since secure API access is optional depending the chip configuration,
fix this by allowing the rest of the code to run even if there's no SMC
device node defined

Signed-off-by: Christian Daudt <csd@broadcom.com>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>