ARM: dts: AM33XX: use pinmux node defined in included file

am33xx boards DTS include the am33xx.dtsi Device Tree
source file that already define a pinmux device node for
the AM33XX SoC Pin Multiplex.

Redefining this for each board makes the Device Tree files
harder to modify and maintain so let's just use what is
already defined in the included .dtsi file.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>