rtc: pl031: switch to rtc_time64_to_tm/rtc_tm_to_time64

Call the 64bit versions of rtc_tm time conversion to allow extending
support after 2106 and properly supporting the STv2 range.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200306005809.38530-3-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>