rtc-m48t59: allow externally mapped ioaddr

Add support for externally mapped ioaddr.  This is required on sparc32
as the ioaddr must be mapped with of_ioremap().

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>