of: Add Device Tree support for SPMI

This change adds SPMI Device Tree parsing. The
of_spmi_register_devices() API should be called from the probe()
routine of each SPMI controller to parse the subtree and add the
respective SPMI devices.

The SPMI subtree is nested up to two levels deep. The first level
is the most basic and treats the address as the SPMI slave ID.
This should be used for simple devices that has no notion of
segmented SPMI address spaces.

An optional second level specifies the address as an offset
within the outer layer's slave ID. This is used to specify
multiple devices on the same slave ID that have different address
ranges. In fact, it's reasonable to specify any number of address
ranges at this level.

Devices can also specify any number of interrupts that's decoding
is done by an external interrupt device.

Sections of this code were taken from drivers/of/platform.c.

Change-Id: Ib9f06764a9bd85e3b2aab43b72aa7132885aa044
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>