ARM: dts: add qcom,pmic-id documentation

The qcom,pmic-id property specifies the PMIC chips
used on a given platform.

This property is used to runtime select a given device tree
blob based on which PMICs are present on a platform. Thus
the device tree with the closest matching qcom,pmic-id will be
selected by the bootloader.

The property consists of a list of 4-tuples which each include
four 32-bit integers denoting the given PMICs in the order of
ascending global SPMI slave id.

If less than four PMICs are present any unused entries must
be populated with 0x0.

A given PMIC cell is defined as follows:
	bits 31-24 = unused
	bits 23-16 = PMIC major version
	bits 15-8  = PMIC minor version
	bits 7-0   = PMIC model number

The closest match is determined by first matching the PMIC
model number, if any model number does not match the given
device the dtb under consideration is discarded.

Subsequently the closest matching major version and minor
version are matched respectively.

Change-Id: Ice3998cd34ac8c318d1b0ca5655b43557b1c63f9
Signed-off-by: David Keitel <dkeitel@codeaurora.org>