[ARM] pxa: update rtc-pxa.c to use 'struct dev_pm_ops'

Remove the following warning:
Platform driver 'pxa-rtc' needs updating - please use dev_pm_ops

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>