Reset controller fixes for v4.11

Fix optional reset_control_get_stubs to return NULL and remove warnings
from reset_control_* stubs.
This fixes commit bb475230b8e5 ("reset: make optional  functions really
optional"), which was merged in reset-for-4.11, and would cause consumer
drivers depending on the new behaviour of optional resets to fail probing
if RESET_CONTROLLER Kconfig option is disabled.
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