mips/malta: load the initrd at the end of the low memory

Currently the malta board is loading the initrd just after the kernel.
This doesn't work for kaslr enabled kernels, as the initrd ends-up being
overwritten.

Move the initrd at the end of the low memory, that should leave a
sufficient gap for kaslr.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Yongbok Kim <yongbok.kim@imgtec.com>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>