arm64: zynqmp: Define default SPL_TEXT_BASE address in Kconfig

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>