omap: Make uncompress code and DEBUG_LL code generic

Define arch_decomp_setup() the same way as some other
architectures do. Use arch_id to configure the debug uart
based on the machine_is by storing it into the uart
scratchpad register for DEBUG_LL code to use.

Note that to avoid merge conflicts, this patch is using
hardcoded register r1 until tmp register is being passed
for all addruart macros.

Signed-off-by: Tony Lindgren <tony@atomide.com>