soc: arm64: intel_socfpga: remove dependency on soc.h

SOCFPGA_SYSMGR_REG_BASE is the only definition from soc.h used in tree,
move it to the file using it. Note that this looks suspicious: base
address should come from Devicetree. This patch will allow to drop
soc.h.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>