fsl-ddr: Allow system to boot if we have more than 4G of memory

Previously if we >=4G of memory and !CONFIG_PHYS_64BIT we'd report
an error and hang.  Instead of doing that since DDR is mapped in the
lowest priority LAWs we setup the DDR controller and the max amount
of memory we report back is what we can map (CONFIG_MAX_MEM_MAPPED)

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Becky Bruce <beckyb@kernel.crashing.org>