genirq: replace irq_gc_ack() with {set,clr}_bit variants (fwd)

This fixes a regression introduced by e59347a1

 "arm: orion:
Use generic irq chip".

Depending on the device, interrupts acknowledgement is done by setting
or by clearing a dedicated register. Replace irq_gc_ack() with some
{set,clr}_bit variants allows to handle both cases.

Note that this patch affects the following SoCs: Davinci, Samsung and
Orion. Except for this last, the change is minor: irq_gc_ack() is just
renamed into irq_gc_ack_set_bit().

For the Orion SoCs, the edge GPIO interrupts support is currently
broken. irq_gc_ack() try to acknowledge a such interrupt by setting
the corresponding cause register bit. The Orion GPIO device expect the
opposite. To fix this issue, the irq_gc_ack_clr_bit() variant is used.

Tested on Network Space v2.

Reported-by: Joey Oravec <joravec@drewtech.com>
Signed-off-by: Simon Guinot <sguinot@lacie.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>