auxdisplay: linedisp: Support configuring the boot message

Like we do for charlcd, allow the configuration of the initial message
on line-display devices.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>