LF-4159-7 arm: dts: imx6ull: 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>