arm: mvebu: db-88f6720: Fix CONFIG_SPL_TEXT_BASE and remove wrong memory layout

Memory layout in the comment is from Armada XP platform which uses load
address 0x40004030. DB-88f6720 is Armada 375 platform which uses same load
address as Armada 38x which is 0x40000030.

Currently SPL support for Armada 375 is unfinished and does not work. There
is missing Serdes initialization and DDR3 training code. So nobody noticed
that CONFIG_SPL_* options are not correct.

Fix at least CONFIG_SPL_TEXT_BASE constant and remove incorrect comments
about memory layout. So it is not misleading.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>