gpio/langwell: clear IRQ edge detect registers at init

The boot firmware might leave the registers configured causing interrupts
to happen even when no handler for them is yet registered. Fix this by
clearing the IRQ edge detect registers at init.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>