led_display: split led display support into generic and hw-dependent parts

Split the display command into generic interface and hardware-specific
realization for PDSP188x LED display found on hmi1001 and manroland
boards. Simple interface for LED displays is defined in
include/led-display.h and described in doc/README.LED_display.
Driver-specific implementation was moved into drivers/misc/pdsp188x.c
file (enabled with CONFIG_PDSP188x set).

Signed-off-by: Ilya Yanok <yanok@emcraft.com>