ARM: uniphier: increase CONFIG_SYS_MONITOR_LEN

With the recent changes, the size of the U-Boot proper image for
uniphier_v7_defconfig exceeded the current limit, 512KB, then SPL
fails to load the whole of the U-Boot proper.  Increase the size.

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