ARM: uniphier: make u-boot-with-spl.bin really available

Commit d085ecd6 ("ARM: uniphier: switch to raw U-Boot image")
claimed that u-boot-with-spl.bin would be useful in its commit log,
but it was not available because the commit missed to define
CONFIG_SPL_MAX_SIZE.  Without it, CONFIG_SPL_PAD_TO is not defined
either (see include/config_fallbacks.h).  So, the SPL image is not
padded correctly.

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