ARM: dts: Add pinctrl macros for dm814x

Let's add the DM814X_IOPAD macro the same way as we have for dm816x and
am33xx as this allows comparing the registers with the documentation easily.
The pinctrl bits are yet again different on dm814x.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>