[ARM] pxa/viper: convert to use plat_serial8250_port irqflags field

Use .irqflags in the plat_serial8250_port structure to set IRQ
polarity, and get rid of the corresponding set_irq_type().

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>