ARM: imx: enable bus auto clock gating function for i.mx6sll

i.MX6SLL has HW bus auto clock gating function, enable
it by default to save VDD_SOC_IN power, about 5% ~ 20%
saved depends on different use cases.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>