i2c-bfin-twi: Add missing pin mux operation

Blackfin TWI controller hardware pin should be requested from GPIO port controller
Before BF54x, there is no need to do this. But as long as BF54x and BF52x
are supported by this generic driver, the missing pin mux operation should be
added.

Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>