mfd: syscon: Use of_iomap() instead of ioremap()

This automatically selects between ioremap() and ioremap_np() on
platforms that require it, such as Apple SoCs.

Signed-off-by: Hector Martin <marcan@marcan.st>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>