dm: pmic: add s2mps11 PMIC I/O driver

This driver allows I/O operations on the Samsung S2MPS11 PMIC,
which provides lots of LDO/BUCK outputs.

To enable it, update defconfig with:
- CONFIG_PMIC_S2MPS11
and additional, if were not defined:
- CONFIG_CMD_PMIC
- CONFIG_ERRNO_STR

The binding info: doc/device-tree-bindings/pmic/s2mps11.txt

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>

doc/device-tree-bindings/pmic/s2mps11.txt

@@ -0,0 +1,17 @@
+SAMSUNG, S2MPS11 PMIC
+
+This file describes the binding info for the PMIC driver:
+- drivers/power/pmic/s2mps11.c
+
+Required properties:
+- compatible: "samsung,s2mps11-pmic"
+- reg = 0x66
+
+With those two properties, the pmic device can be used for read/write only.
+
+Example:
+
+s2mps11@66 {
+	compatible = "samsung,s2mps11-pmic";
+	reg = <0x66>;
+};