The upstream device tree no longer lists "syscfg-clk" and the syscon node
contains a clocks property that the barebox syscon driver now
automatically clocks before access.

Thus drop the requirement that "syscfg-clk" is specified.

Fixes: 4aaceef58 ("dts: update to v5.6-rc1")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>