arm64: defconfig: enable CONFIG_LOCALVERSION_AUTO

When CONFIG_LOCALVERSION_AUTO is disabled, the version string is
just a tag name (or with a '+' appended if HEAD is not a tagged
commit).

During the development (and especially when git-bisecting), longer
version string would be helpful to identify the commit we are running.

This is a default y option, so drop the unset to enable it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>