MIPS: malta: Use img-ascii-lcd driver for LCD display

Remove the Malta display platform code in favour of probing the
img-ascii-lcd driver via device tree. This reduces the amount of
platform code & the img-ascii-lcd driver offers us advantages in terms
of code sharing with other boards & functionality such as changing the
displayed message via sysfs. Defconfigs are untouched because the driver
already defaults y on when CONFIG_MIPS_MALTA=y.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21182/
Cc: linux-mips@linux-mips.org