ARM: dts: AM3XXX: Use #include for all device trees

Replace /include/ by #include for AM33XX and AM35XX device tree
files, in order to use the C pre-processor, making use of #define
features possible.

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>