of: unittest: Fix build on architectures without CONFIG_OF_ADDRESS

of_dma_get_max_cpu_address() is not defined if !CONFIG_OF_ADDRESS, so
return early in of_unittest_dma_get_max_cpu_address().

Fixes: 07d13a1d ("of: unittest: Add test for of_dma_get_max_cpu_address()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>