ARM: dts: AM33XX: Specific pinctrl header

The pinctrl IP inside the AM33XX family differs slightly from
what is found on OMAP2+. Define a specific header to take account
of the differences.

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>