Get rid of the attb_read_val() platform hook. Instead, read the ATTB gpio
directly from the driver.

Fail if valid ATTB gpio is not provided by patform data.
Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>