rtc: mt2712: switch to devm_platform_ioremap_resource

Using devm_platform_ioremap_resource instead of open coding it reduces the
size of the binary.

   text	   data	    bss	    dec	    hex	filename
   3728	    216	      0	   3944	    f68	drivers/rtc/rtc-mt2712.o
   3744	    216	      0	   3960	    f78	drivers/rtc/rtc-mt2712.o.old

Link: https://lore.kernel.org/r/20200415200021.157118-2-alexandre.belloni@bootlin.com


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