mfd: palmas: Fix resource leak of i2c_dummy devices

Palmas device supports multiple i2c device address and the client
for these addressed are created in the driver as i2c_new_dummy().

The new devices are not getting released in error or removal path and
so it is causing resource leak.

Add the unregister of these newly created dummy devices to avoid resource
leaks.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>