ARM: dts: AM33XX: Add default pinctrl binding for UART0 device

Add pin control binding for UART0 device nodes in all
board specific DT files.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Acked-by: Matt Porter <mporter@ti.com>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>