drivers: of: fix build break if asm/dma-contiguous.h is missing

It is not needed to include asm/dma-contiguous.h header to compile
reserved memory initialization code, so remove it to avoid build break
on architectures without CMA support.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>