ARM: dts: i.MX25: define SSI FIFO depth

According to the i.MX25 reference manuals, each SSI has four FIFOs. All
of those FIFOs can store up to 15 entries.

The fsl_ssi driver's internal default for the FIFO depth in 8. Set our
non-default FIFO depth explicitly in the Device Tree.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>