sconfig: Allow to link devices to other device's drivers

Rarely, the driver of one device needs to know about another device
that can be anywhere in the device hierarchy. Current applications
boil down to EEPROMs that store information that is consumed by some
code (e.g. MAC address).

The idea is to give device nodes in the `devicetree.cb` an alias that
can later be used to link it to a device driver's `config` structure.
The driver has to declare a field of type `struct device *`, e.g.

    struct some_chip_driver_config {
            DEVTREE_CONST struct device *needed_eeprom;
    };

In the devicetree, the referenced device gets an alias, e.g.

    device i2c 0x50 alias my_eeprom on end

The author of the devicetree is free to choose any alias name that
is unique in the devicetree. Later, when configuring the driver the
alias can be used to link the device with the field of a driver's
config:

    chip some/chip/driver
            use my_eeprom as needed_eeprom
    end

Override devices can add an alias if it does not exist, but cannot
change the alias for a device that already exists.

Alias names are checked for conflicts both in the base tree and in the
override tree.

References are resolved after the tree is parsed so aliases and
references do not need to be in a specific order in the tree.

Change-Id: I058a319f9b968924fbef9485a96c9e3f900a3ee8
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35456
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/sconfig/sconfig.h

@@ -57,6 +57,9 @@ struct chip_instance {
 	/* Pointer to registers for this chip. */
 	struct reg *reg;
 
+	/* Pointer to references for this chip. */
+	struct reg *ref;
+
 	/* Pointer to chip of which this is instance. */
 	struct chip *chip;
 
@@ -123,6 +126,9 @@ struct device {
 	/* Name of this device. */
 	char *name;
 
+	/* Alias of this device (for internal references) */
+	char *alias;
+
 	/* Path of this device. */
 	char *path;
 	int path_a;
@@ -172,7 +178,7 @@ extern struct bus *root_parent;
 struct device *new_device(struct bus *parent,
 			  struct chip_instance *chip_instance,
 			  const int bustype, const char *devnum,
-			  int status);
+			  char *alias, int status);
 
 void add_resource(struct bus *bus, int type, int index, int base);
 
@@ -195,6 +201,7 @@ void *chip_dequeue_tail(void);
 
 struct chip_instance *new_chip_instance(char *path);
 void add_register(struct chip_instance *chip, char *name, char *val);
+void add_reference(struct chip_instance *chip, char *name, char *alias);
 
 struct fw_config_field *get_fw_config_field(const char *name);