ARM: uniphier: move SPL stack address

Currently, the address region, 0xf8000 - 0x100000, is used for SPL
stack for the 32bit SoCs.  Because the U-Boot proper image starts
from 0x70000, the maximum size of the U-Boot proper image is 544KB
(0x70000 - 0xf8000) for the NOR boot mode.  Now uniphier_v7_defconfig
is almost hitting this size limit.  Changing CONFIG_SPL_STACK can
raise the size limit with less impact.  With this, the size limit
will increase to 576KB (0x70000 - 0x100000).  If we need to increase
it even more, we would be able to change CONFIG_SYS_UBOOT_BASE at the
cost of the flashing command changes.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>