pwm: pca9685: depends on I2C rather than REGMAP_I2C

REGMAP_I2C is not a visible config option.
Thus make PWM_PCA9685 depend on I2c and then select REGMAP_I2C.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>