[ARM] i.MX serial: fix IRQ allocation

If RTS interrupt is caused by RTS senzing logic inside i.MX UART module the
IRQ type cannot be set.

It applies only for interrupts going through GPIO layer.  The problem has
been noticed by Konstantin Kletschke some time ago.

  No IRQF_TRIGGER set_type function for IRQ 26 (MPU)

I would not change type to fixed 0, because it could be possible to use
different GPIO MX1 pin for RTS in the theory.  On the other hand it is only
for documentation purposes now, because RTS read code would have to be
adjusted in such case.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>