ARM: dts: kirkwood: lsxl: fix serial line

ARM: dts: kirkwood: lsxl: fix serial line

Commit 327e15428977 ("ARM: dts: kirkwood: consolidate common pinctrl
settings") unknowingly broke the serial output on this board. Before
this commit, the pinmux was still configured by the bootloader and the
kernel didn't reconfigured it again. This was an oversight by the
initial board support where the pinmux for the serial line was never
configured by the kernel. But with this commit, the serial line will be
reconfigured to the wrong pins. This is especially confusing, because
the output still works, but the input doesn't. Presumingly, the input is
reconfigured to MPP10, but the output is connected to both MPP11 and
MPP5.

Override the pinmux in the board device tree.

Fixes: 327e15428977 ("ARM: dts: kirkwood: consolidate common pinctrl settings")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>