hwrng: imx-rngc - make symbol imx_rngc_pm_ops static

Fixes the following sparse warnings:

drivers/char/hw_random/imx-rngc.c:303:1: warning:
 symbol 'imx_rngc_pm_ops' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>