rtc: pxa: fix possible race condition

pxa_rtc_open() registers the interrupt handler which will access the RTC
registers. However, pxa_rtc_open() is called before the register range is
ioremapped. Instead, call it after devm_ioremap().

Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>