arm64: include EXT4 in defconfig

Most readily available root filesystems are formatted as EXT4 these
days. For example, see the raring rootfs that the Debian folk is
preparing [1].

[1]: http://people.debian.org/~wookey/bootstrap/rootfs/

Cc: Will Deacon <will.deacon@arm.com>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>