Problem reported by Thomas Skibo (ARM mailinglist)
My interpretation of Michal Meloun proposal of solution to the problem.
Add support for simplebus in syscon_generic.c.
To handle the syscon in TI am335x device tree the driver also need to have the clkdev interface (write/read/modify...). For example sys_clkin_ck@40 (line 8 in am33xx-clocks.dtsi) need the clkdev interface.