lib/reg_script: Add display support

Add the ability to enable the display of the script:
* Added REG_SCRIPT_COMMAND_DISPLAY to enable and disable display output
* Added context values to manage display support
  * display_state - Updated by the command to enable or disable display
  * display_features - May be updated by step routine to control what
    the step displays for register and value
  * display_prefix - Prefix to display before register data
* Added REG_SCRIPT_DISPLAY_ON and REG_SCRIPT_DISPLAY_OFF macros to
  control the display from the register script
* Added REG_SCRIPT_DISPLAY_REGISTER and REG_SCRIPT_DISPLAY_VALUE as
  two features of the common display.  With these features enabled
  the following is output:
  * Write: <optional prefix> register <-- value
  * Read:  <optional prefix> register --> value

TEST=Build and run on Galileo Gen2

Change-Id: If0d4d61ed8ef48ec20082b327f358fd1987e3fb9
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/14553
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/reg_script.h

@@ -47,6 +47,10 @@ enum {
 	REG_SCRIPT_COMMAND_POLL,
 	REG_SCRIPT_COMMAND_SET_DEV,
 	REG_SCRIPT_COMMAND_NEXT,
+	REG_SCRIPT_COMMAND_DISPLAY,
+
+	/* Insert new types above this comment */
+
 	REG_SCRIPT_COMMAND_END,
 };
 
@@ -92,6 +96,9 @@ struct reg_script_context {
 	device_t dev;
 	struct resource *res;
 	const struct reg_script *step;
+	uint8_t display_state;    /* Only modified by reg_script_run_step */
+	uint8_t display_features; /* Step routine modifies to control display */
+	const char *display_prefix; /* Prefix tag to display */
 };
 
 struct reg_script_bus_entry {
@@ -132,6 +139,21 @@ struct reg_script_bus_entry {
 	  .res_index = res_index_, \
 	}
 
+/* Display control */
+#define REG_SCRIPT_DISPLAY_ALL		0xff
+#define REG_SCRIPT_DISPLAY_REGISTER	0x02
+#define REG_SCRIPT_DISPLAY_VALUE	0x01
+#define REG_SCRIPT_DISPLAY_NOTHING	0
+
+#define REG_SCRIPT_DISPLAY_OFF                   \
+	{ .command = REG_SCRIPT_COMMAND_DISPLAY, \
+	  .value = REG_SCRIPT_DISPLAY_NOTHING,   \
+	}
+#define REG_SCRIPT_DISPLAY_ON                    \
+	{ .command = REG_SCRIPT_COMMAND_DISPLAY, \
+	  .value = REG_SCRIPT_DISPLAY_ALL,       \
+	}
+
 /*
  * PCI
  */