bus: ti-sysc: Fix a build warning with W=1 for sysconfig

Dhruva reported a build warning with W=1 for "Function parameter or member
'sysconfig' not described in 'sysc'". Let's document sysconfig to fix this.

Reported-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>