dm: pmic: add implementation of driver model pmic uclass

This commit introduces the PMIC uclass implementation.
It allows providing the basic I/O interface for PMIC devices.
For the multi-function PMIC devices, this can be used as I/O
parent device, for each IC's interface. Then, each PMIC particular
function can be provided by the child device's operations, and the
child devices will use its parent for read/write by the common API.

Core files:
- 'include/power/pmic.h'
- 'drivers/power/pmic/pmic-uclass.c'

The old pmic framework is still kept and is independent.

For more detailed informations, please look into the header file.

Changes:
- new uclass-id: UCLASS_PMIC
- new config: CONFIG_DM_PMIC

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>

include/dm/uclass-id.h

@@ -49,6 +49,9 @@ enum uclass_id {
 	UCLASS_USB_DEV_GENERIC,	/* USB generic device */
 	UCLASS_USB_HUB,		/* USB hub */
 
+	/* Power Management */
+	UCLASS_PMIC,		/* PMIC I/O device */
+
 	UCLASS_COUNT,
 	UCLASS_INVALID = -1,
 };