mfd/tc3589x: add block identifier for multiple child devices

Add block identifier to be able to add multiple mfd clients
to the mfd core

Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Sundar Iyer <sundar.iyer@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>