omap: Fix DEBUG_LL uart to access phys addr when MMU isn't enable

Now we use a memory address to store the debug port info, So we need
to read/write this address when we choose DEBUG_LL. When MMU isn't
enable(I.E. the begining part of init stage of the linux kernel boot),
we need to access physical address instead of virtual address,
otherwise the kernel will crash.

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>