bus: imx-weim: make symbol 'weim_of_notifier' static

The sparse tool complains as follows:

drivers/bus/imx-weim.c:373:23: warning:
 symbol 'weim_of_notifier' was not declared. Should it be static?

This symbol is not used outside of imx-weim.c, so marks it static.

Fixes: e6cb5408 ("bus: imx-weim: add DT overlay support for WEIM bus")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>