powerpc/mpc85xx: fix fsl/p2041-post.dtsi clockgen mux2

The mux2 node is missing the clock-output-names field that is required
by the clk-ppc-corenet driver.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>