ARM: dts: AM33XX: Use pinctrl constants

Using constants for pinctrl allows a better readability, and removes
redundancy with comments.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Tested-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>