serial: mvebu-uart: dissociate RX and TX interrupts

While the standard UART port can use a single IRQ that 'sums' both RX
and TX interrupts, the extended port cannot and has to use two different
ISR, one for each direction. The standard port also has the hability
to use two separate interrupts (one for each direction).

The logic is then: either there is only one unnamed interrupt on the
standard port and this interrupt must be used for both directions
(this is legacy bindings); or all the interrupts must be described and
named 'uart-sum' (if available), 'uart-rx', 'uart-tx' and two separate
handlers for each direction will be used.

Suggested-by: Allen Yan <yanwei@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>