OMAP: Zoom2: update serial platform_data id for external UART

Now that on-chip UARTs each have separate platform_data, the external
UART needs an non-conflicting ID.  Since there are 3 on-chip UARTs,
the Zoom2 external UART will be registered after as the fourth.

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>