dt-bindings: Add pxa1928 clock binding

This adds the clock binding documentation for the Marvell PXA1928 SOC.
The PXA1928 has 3 clock control blocks for different subsystems of the
chip.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>