ARM: exynos: fix l2x0 saved regs handling

The exynos4_l2x0_cache_init function tries to flush the data cache
for the location of the saved l2x0 registers and pass the physical
address to the s5p-sleep implementation.

However, the s5p-sleep code is optional, and if it is disabled,
we get a linker error here when the l2x0_regs_phys variable does
not exist.

To solve this, use a compile-time conditional to drop this code
if we don't want it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Ben Dooks <ben-linux@fluff.org>