Qualcomm SPMI arbiter/bus driver

This is bus driver for Qualcomm chips that use SPMI to communicate with PMICs.

Required properties:
- compatible: "qcom,spmi-pmic-arb"
- reg: Register block adresses and sizes for various parts of device:
   1) PMIC arbiter channel mapping base (PMIC_ARB_REG_CHNLn)
   2) SPMI write command (master) registers (PMIC_ARB_CORE_SW_DEC_CHANNELS)
   3) SPMI read command (observer) registers (PMIC_ARB_CORE_REGISTERS_OBS)

Optional properties (if not set by parent):
- #address-cells: 0x1 - childs slave ID address
- #size-cells: 0x1

All PMICs should be placed as a child nodes of bus arbiter.
Automatic detection of childs is currently not supported.

Example:

spmi@200f000 {
	compatible = "qcom,spmi-pmic-arb";
	reg = <0x200f800 0x200 0x2400000 0x400000 0x2c00000 0x400000>;
	#address-cells = <0x1>;
	#size-cells = <0x1>;
};