arm: include: Add DTS generated file to arch.h

This patch moves the include for the generated_dts_board.h inside of
the include/arch/arm/arch.h file.  This was done to simplify the
includes required for files.  Only two files will include the dts
generated include file directly: arch.h and the linker.ld

Change-Id: I2614f4fd4eeed2ab635a3264d7dac8b83f97b760
Signed-off-by: Andy Gross <andy.gross@linaro.org>