soc: bcm: brcmstb: pm: pm-arm: Provide prototype for brcmstb_pm_s3_finish()

brcmstb_pm_s3_finish() cannot be made static because it is referenced
from brcmstb_pm_s3(), so let's provide a prototype for it instead.

Fixes the following W=1 kernel build warning(s):

 drivers/soc/bcm/brcmstb/pm/pm-arm.c:395:14: warning: no previous prototype for ‘brcmstb_pm_s3_finish’ [-Wmissing-prototypes]

Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>