arm64: dts: renesas: white-hawk-cpu: Fix missing serial console pin control

The pin control description for the serial console was added, but not
enabled, due to missing pinctrl properties in the serial port device
node.

Fixes: 7a8d590d ("arm64: dts: renesas: white-hawk-cpu: Add serial port pin control")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/8a51516581cd71ecbfa174af9c7cebad1fc83c5b.1702459865.git.geert+renesas@glider.be