This change allows to reserve enough space at the end of board SDRAM
to store two copies of U-Boot and malloc heap.

Due to selection of the CONFIG_SYS_TEXT_BASE the second code/data
copying is not avoided, first of all this may depend on a used
toolchain, secondly at this point some level of volatility is wanted
to do more platform changes and do not care about probably changed
calculated in runtime relocation offset.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>