cros_ec: Support the LDO access method used by spring

Add a driver to support the special LDO access used by spring. This is a
custom method in the cros_ec protocol - it does not use an I2C
pass-through.

There are two implementation choices:

1. Write a special LDO driver which can talk across the EC. Duplicate all
the logic from TPS65090 for retrying when the LDO fails to come up.

2. Write a special I2C bus driver which pretends to be a TPS65090 and
transfers reads and writes using the LDO message.

Either is distasteful. The latter method is chosen since it results in less
code duplication and a fairly simple (30-line) implementation of the core
logic.

The crosec 'ldo' subcommand could be removed (since i2c md/mw will work
instead) but is retained as a convenience.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/cros_ec.h b/include/cros_ec.h
index 41951c3..0ad9d81 100644
--- a/include/cros_ec.h
+++ b/include/cros_ec.h
@@ -350,7 +350,7 @@
  * @param state		new state of the LDO/FET : EC_LDO_STATE_ON|OFF
  * @return 0 if ok, -1 on error
  */
-int cros_ec_set_ldo(struct cros_ec_dev *dev, uint8_t index, uint8_t state);
+int cros_ec_set_ldo(struct udevice *dev, uint8_t index, uint8_t state);
 
 /**
  * Read back a LDO / FET current state.
@@ -360,7 +360,7 @@
  * @param state		current state of the LDO/FET : EC_LDO_STATE_ON|OFF
  * @return 0 if ok, -1 on error
  */
-int cros_ec_get_ldo(struct cros_ec_dev *dev, uint8_t index, uint8_t *state);
+int cros_ec_get_ldo(struct udevice *dev, uint8_t index, uint8_t *state);
 
 /**
  * Get access to the error reported when cros_ec_board_init() was called