powerpc: add fsl,fifo-depth property to Freescale SSI device nodes

The Freescale Serial Synchronous Interface (SSI) is an audio device present on
some Freescale SOCs.  Various implementations of the SSI have a different
transmit and receive FIFO depth, but are otherwise identical.  To support
these variations, add a new property fsl,fifo-depth to the SSI node that
specifies the depth of the FIFOs.

Also update the MPC8610 HPCD device tree with this property.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>