Linux supports a clocks property in syscon nodes to indicate that access
should only occur with the clock active. The new regmap-mmio supports
this, thus port syscon over, so it can make use of it in a later commit.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>