ARM: EXYNOS: Do not resume l2x0 if not enabled before suspend

Trying to resume l2x0 if it was not enabled before suspend leads to
system crash. This patch prevents this by checking if l2x0_regs_phys is
a valid pointer to l2x0 context data saved on initialization.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>