While I was testing irq's on the cubietruck I found a couple of
not working irq pins. Further diving into the problem it opened
up a mess called "manual".

This so called manual (A20 user manual v1.3 dated 2014-10-10) says:

Pin overview:
    Page 237:       EINT26 is on mux 5.
    Page 288:       EINT26 is on mux 6.

The manual is so contradicting that further tests had to be made
to see which of the 2 statements where correct.

This patch is based on actual outcome of these tests and not what
the manual says.

Test procedure used:

Connect a 1 pulse per second (GPS) line to the pin.

echo pin### > /sys/class/gpio/export
echo in > /sys/class/gpio/gpio###/direction
echo rising > /sys/class/gpio/gpio###/edge

Check /proc/interrupts if a irq was attached and if irq's where
received.

Hardware used:
Henry Paulissen: Cubietruck
Andere Przywara: BananaPi M1
Tested-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Henry Paulissen <henry@nitronetworks.nl>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>