fsl/deepsleep: avoid the DDR restore from being optimized out

Function dp_ddr_restore is to restore the first 128-byte space
of DDR. However those codes may be optimized out by compiler
since the destination address is at 0x0. In order to avoid
compiler optimization, we restore the space from high address,
which is not at 0x0, to low address.

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>