ARM: da850-evm: Fix boot issues from missing SPL_PAD_TO

ARM: da850-evm: Fix boot issues from missing SPL_PAD_TO

In a previous attempt to unify config options and remove items
from the whitelist file, SPL items were moved into a section
enabled with CONFIG_SPL_BUILD.  Unfortunately, SPL_PAD_TO
is referenced at the head Makefile and uses this define
to create padding of the output file.  When it was moved
to CONFIG_SPL_BUILD, it caused boot errors with devices
that are not booting from NOR.  Fix the boot issues by moving
SPL_PAD_TO out so it's always.

Fixes: 7bb33e4684aa ("ARM: da850-evm: Unify config options with Kconfig")
Signed-off-by: Adam Ford <aford173@gmail.com>