arm: mvebu: Move CONFIG_SPL_LDSCRIPT to common header

This way, new MVEBU boards don't need to specifiy the common location
for the SPL linker script.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Tested-by: Dirk Eibach <dirk.eibach@gdsys.cc>