mx31pdk: Move CONFIG_SPL_LDSCRIPT to defconfig

As there is only one mx31pdk config file and with upcoming updates to
the Kconfig parsing logic, rather than have an entry in
board/freescale/mx31pdk/Kconfig, move this single setting to the
defconfig file.

Cc: Magnus Lilja <lilja.magnus@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Magnus Lilja <lilja.magnus@gmail.com>