ARM: multi_v7_defconfig: enable UniPhier UART driver

The UniPhier on-chip UART driver was merged into the mainline by
commit 1a8d2903 ("serial: 8250_uniphier: add UniPhier serial
driver").

Enable it from multi_v7_defconfig.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Olof Johansson <olof@lixom.net>