rtc: 88pm80x: add device tree support

Along with DT support, this patch also cleans up the unnecessary
code around 'rtc_wakeup' initialization.

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>