power/reset: make syscon_poweroff() static

The syscon_poweroff() function is not exported or declared for
usage elsewhere, so make it static to avoid the folloiwing warning:

drivers/power/reset/syscon-poweroff.c:33:6: warning: symbol 'syscon_poweroff' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>