Blackfin serial driver: add extra IRQ flag for 8250 serial driver

Signed-off-by: NJavier Herrero <jherrero@hvsistemas.es>
Signed-off-by: NBryan Wu <cooloney@kernel.org>

include/asm-blackfin/serial.h

+/*
+ * include/asm-blackfin/serial.h
+ */
+
+#define SERIAL_EXTRA_IRQ_FLAGS IRQF_TRIGGER_HIGH