OMAP3: serial - allow platforms specify which UARTs to initialize

This patch adds new function: omap_serial_init_port(port) that can be
used to initialize only selected UARTs as serial ports. Platforms can
then in their board files call this function instead of omap_serial_init()
if they don't want to use all UARTs as serial ports.

Signed-off-by: Mika Westerberg <ext-mika.1.westerberg@nokia.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>