ARM: uniphier: enable MTD partition and UBI

Enable "mtdparts" and "ubi" commands for uniphier_v8_defconfig to
use UBI on NAND devices.

Enable only "mtdparts" for uniphier_{v7,ld4_sld8}_defconfig because
enabling UBI would increase 170KB, which would be memory footprint
problem.

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