mfd: twl6040: The chip does not support bulk access

Bulk access is not working with twl6040, we need to use single register
access. Bulk access would happen when we try to sync the regcache after
power on.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>