omap3: zoom 2/3: Change debugboard serial port id

This is now changed to PLAT8250_DEV_PLATFORM (= 0), because
it's the only port that's going to be initialized in
Zoom 2/3 boards.

So, it doesn't make sense to keep the hardcoded 3 value anymore.

Signed-off-by: Sergio Aguirre <saaguirre@ti.com>