i2c: designware_i2c: Add dw_i2c_enable() helper function

i2c: designware_i2c: Add dw_i2c_enable() helper function

dw_i2c_enable() is used to dis-/en-able the I2C controller. It makes
sense to add such a function, as the controller is dis-/en-abled
multiple times in the code. Additionally, this function now checks,
if the controller is really dis-/en-abled. This code is copied
from the Linux I2C driver version.

Signed-off-by: Stefan Roese <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Bin Meng <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Heiko Schocher <[email protected]>
Reviewed-by: Bin Meng <[email protected]>