[POWERPC] fsl: Update fsl_soc to use cell-index property of I2C nodes

Currently, fsl_i2c_of_init() uses the order of the I2C adapter nodes in the
device tree to enumerate the I2C adapters.  Instead, let's check for the
cell-index property and use it if it exists.

This is handy for device drivers that need to identify the I2C adapters by
specific numbers.  The Freescale MPC8610 ASoC V2 sound drivers are an example.

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