pinctrl: pinctrl-imx: Print the mux_mode field in hex format

With debug enabled we get better readability dumps of the mux_mode register if
we use hexadecimal format instead:

imx6sl-pinctrl 20e0000.iomuxc: MX6SL_PAD_FEC_REF_CLK: 0x10 0x0001b0a8

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>