ARM: uniphier: define CONFIG_SYS_BOOTMAPSZ

U-Boot relocates the device tree and the initramdisk to the tail
of the memory region before booting the kernel.

Some UniPhier boards are equipped with a large amount of memory.
For those boards, the device tree and the initramdisk are placed out
of the the kernel causing a kernel panic.

Add CONFIG_SYS_BOOTMAPSZ to prevent them from going too high.

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