clk: imx6ul: fix periph clk2 clock mux selection

According to the data sheet the 3rd choice is the bypass clock
of pll2. This should not have any effect in practice as this
selection is not used currently.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>