cmd: mem: Remove CONFIG_SYS_MEMTEST_SCRATCH mapping

cmd: mem: Remove CONFIG_SYS_MEMTEST_SCRATCH mapping

There is no real need to exactly define space for saving patterns for
alternate memory test. It is much easier to allocate space on the stack and
use it instead of trying to find out space where pattern should be saved.

For example if you want to test the whole DDR memory you can't save patter
to DDR and you need to find it out. On Xilinx devices DDR or OCM addresses
were chosen but that means that OCM needs to be mapped and U-Boot has
access permission there.

It is easier to remove this limitation and simply save it on stack because
it is very clear that memory test can't rewrite U-Boot and U-Boot has also
full access to memory where runs from.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>