imx: Explicitly pass the I2C bus number in pmic_init()

The pmic_init() function has the I2C or SPI bus number that is connected to the
PMIC.

Instead of passing I2C_PMIC, explicitly pass the I2C bus number via I2C_x
definition.

The motivation for doing this is to avoid people just doing a copy and paste
of I2C_PMIC into their board file when another I2C bus is actually used to
interface to their PMIC.

This also makes more obvious which is the I2C bus connected to the PMIC, without
having to search in the source code for the meaning of the 'I2C_PMIC' number.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>