mfd/tc35892: rename tc35892 core driver to tc3589x

Rename the tc35892 core/gpio drivers to tc3589x to include
new variants in the same 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>