serial: mvebu-uart: initialize over sampling stack register

The baudrate derivation relies on the state of the programmable over
sampling stack (OSAMP register) being empty, while never initializing
it.

Set all the fields of this register to 0 (except reserved areas) to
ensure a x16 divisor, as assumed by the driver.

The suspend/resume callbacks are untouched because they already
save/restore correctly this register.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>