MLK-20956 rtc: imx-rpmsg: do NOT overwrite rtc driver data

Some test cases need to use RTC driver data, so do NOT
overwrite it using rpmsg data structure.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Bai Ping <ping.bai@nxp.com>