On platforms that mix dedicated IP block register space with
miscellaneous registers it is necessary to share register window between
syscon and dedicated IP block driver. Calling request_iomem_region()
implies exclusive ownership of the region, which, in the case above
could not happen.

This change also makes this driver's behaviour to that of its Linux
kernel counterpart.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>