ARM: EXYNOS: Move SYSREG definition into sys-reg specific file

While making PMU implementation to be device tree based, there are
few register offsets related with SYSREG present in regs-pmu.h, so
let's make a new header file "regs-sys.h" to keep all such SYSREG
related register offsets and remove them from "regs-pmu.h"

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>