mpc8315erdb: Merge BR/OR settings

mpc8315erdb: Merge BR/OR settings

The mpc8315erdb has the option of either configuring the eLBC (enhanced
local system bus) such that

* NOR flash is the first memory bank, and NAND flash is the second
  memory bank, or
* NAND flash is the first memory bank, and NOR flash is the second
  memory bank,

by using CONFIG_SYS_NOR_{BR,OR}_PRELIM and
CONFIG_SYS_NAND_{BR,OR}_PRELIM for defining
CONFIG_SYS_{BR,OR}{0,1}_PRELIM.

After Kconfig migration, replacing some lines in the defconfig will have
the same effect.

Hence, we will not create distinct ve8313_{NOR,NAND} configs for such a small change.

Instead, fix the current default (NOR first, NAND second), and unroll
the CONFIG_SYS_NAND_{BR,OR}_PRELIM options. This will ease the Kconfig
migration.

Signed-off-by: Mario Six <mario.six@gdsys.cc>