ARM: socfpga: Factor out handoff register configuration

ARM: socfpga: Factor out handoff register configuration

Factor out the code for programming preloader handoff register values,
the ISWGRP Handoff 0 and 1. These registers later control which bridges
are enabled by the "bridge" command on Gen5 devices.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <chin.liang.see@intel.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Tien Fong Chee <tien.fong.chee@intel.com>