rtc: sprd: Add new RTC power down check method

We should use the new method to check if RTC was powered down, which
is more solid. Since we have introduced power control and power status
registers, and we just check if the power status is the default value
(0x96), if yes that means the RTC has been powered down. Meanwhile We
can set the power control register to be one valid value to change
the power status to indicate RTC device is valid now.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>