omap: introduce OMAP_LL_DEBUG_NONE DEBUG_LL config

Zoom2/Zoom3 kind of boards do not use omap uarts for console.
These use external debug board for console.

So these boards should not have "Uncompressing Kernel...."
log put on omap uarts.

By interoducing OMAP_LL_DEBUG_NONE option,
unnecessary writes to omap uarts is avoided.

In future, the DEBUG_LL interface will be enhanced
to use external debug board.

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>