Davinci: generalized debug macros

This patch adopts a debug uart selection similar to the OMAP model.  During
the boot process, the uncompress code determines the physical and virtual base
addresses of the board-specific debug uart.  These addresses are then passed
on to the in-kernel debug macros through a small chunk of memory placed just
below the page tables (@0x80003ff8).

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>