Add support for setting the I2C bus speed in fsl_i2c.c

Add support to the Freescale I2C driver (fsl_i2c.c) for setting and querying
the I2C bus speed.  Current 8[356]xx boards define the CFG_I2C_SPEED macro,
but fsl_i2c.c ignores it and uses conservative value when programming the
I2C bus speed.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Andy Fleming <afleming@freescale.com>