Davinci: serial - remove unnecessary define

The uart pdata array is already terminated by a zero flag field.

This patch reuses this terminator and eliminates DAVINCI_MAX_NR_UARTS
definition.  This way, future platforms can have different number of uarts
initialized via davinci_serial_init().

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