ARM: dts: overo: Push uart3 pinmux down to expansion board

UART3 is used by expansion boards to get a tty console. Thus, the
pinmux should be defined by expansion boards, instead of being
imposed by the processor board.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Tony Lindgren <tony@atomide.com>