LF-4159-6 arm: dts: imx6sll: add dummy clock for epdc_ahb

add dummy clock for epdc_ahb as it shares epdc driver with imx8ulp

Signed-off-by: Robby Cai <robby.cai@nxp.com>
Reviewed-by: G.n. Zhou <guoniu.zhou@nxp.com>