Davinci: serial - conditional reset via pwremu

With this patch, AR7 type uart ports are not reset via pwremu registers.  This
allows davinci_serial_init() reuse on tnetv107x soc.

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