qemuarm64.conf: Make the second serial console /dev/hvc0

qemuarm64.conf: Make the second serial console /dev/hvc0

Since the qemu for aarch64 must use a virtual console for the second
serial port rather than emulating actual hardware, make sure the correct
device is specified so that a tty is actually started.

Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>