soc: arm64: intel_socfpga: changes in system_manager

SOCFPGA_SYSMGR_REG_BASE base address now read from Device tree

This commit changes the way the SOCFPGA_SYSMGR_REG_BASE base address is
determined. Previously, the address was hard-coded in the system_manager
source file. This commit changes the code to read the address from the
Device tree. This makes the code more flexible and allows the base
address to be different for different boards.

Signed-off-by: Navinkumar Balabakthan <navinkumar.balabakthan@intel.com>