sandbox: dts: add sandbox_pmic.dtsi and include it to sandbox.dts and test.dts

sandbox: dts: add sandbox_pmic.dtsi and include it to sandbox.dts and test.dts

This commit adds dtsi file for Sandbox PMIC.
It fully describes the PMIC by:
- i2c emul node - with a default settings of 16 registers
- 2x buck regulator nodes
- 2x ldo regulator nodes

The default register settings are set with preprocessor macros:
- VAL2REG(min[uV/uA], step[uV/uA], val[uV/uA])
- VAL2OMREG(mode id)
Both defined in file:
- include/dt-bindings/pmic/sandbox_pmic.h

The Voltage ranges of each regulator can be found in:
- include/power/sandbox_pmic.h

The new file is included into:
- sandbox.dts
- test.dts

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