auxdisplay: linedisp: Add support for changing scroll rate

If the message to display is longer than the number of characters that
the display can show, the message will be scrolled.  Currently the
scroll rate is fixed, moving every 500 ms.

Add support for changing the scroll rate through a "scroll_step_ms"
device attribute in sysfs.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>