Define default address via Kconfig. There is no need to change this address
for most of the boards but it is also possible. This one line save a lot of
lines in defconfigs that's why make sense to do it.

The similar change has been done by commit 9340d8fe
("sunxi: move CONFIG_SPL_TEXT_BASE from *_defconfig to Kconfig")

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>