treewide: mem: Move mtest related defines to Kconfig

Move below defines which are used by mtest utility to Kconfig.
CONFIG_SYS_MEMTEST_START
CONFIG_SYS_MEMTEST_END

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
[trini: Fix kmcoge5ne board, re-run migration as well]
Signed-off-by: Tom Rini <trini@konsulko.com>