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>

doc/README.LED_display

@@ -0,0 +1,27 @@
+LED display internal API
+=======================================
+
+This README describes the LED display API.
+
+The API is defined by the include file include/led-display.h
+
+The first step in to define CONFIG_CMD_DISPLAY in the board config file.
+Then you need to provide the following functions to access LED display:
+
+void display_set(int cmd);
+
+This function should control the state of the LED display. Argument is
+an ORed combination of the following values:
+ DISPLAY_CLEAR	-- clear the display
+ DISPLAY_HOME	-- set the position to the beginning of display
+ DISPLAY_MARK	-- enable mark (decimal point), if implemented
+
+int display_putc(char c);
+
+This function should display it's parameter on the LED display in the
+current position. Returns the displayed character on success or -1 in
+case of failure.
+
+With this functions defined 'display' command will display it's
+arguments on the LED display (or clear the display if called without
+arguments).