arm: socfpga: Convert system manager from struct to defines

Convert system manager for Gen5, Arria 10 and Stratix 10 from struct
to defines.

Change to get system manager base address from DT node instead of
using #define.

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>