of: Add function for generating a DT modalias with a newline

The modalias sysfs attr is lacking a newline for DT aliases on platform
devices. The macio and ibmebus correctly add the newline, but open code it.
Introduce a new function, of_device_modalias(), that fills the buffer with
the modalias including the newline and update users of the old
of_device_get_modalias function.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: linuxppc-dev@lists.ozlabs.org
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>