staging/iio/lis3l02dq: fix building without irq_to_gpio

The driver has not been building for some time after the
irq_to_gpio function has been removed from the kernel.

The only board in the upstream kernel that provides
this device is the "Stargate 2", which is also maintained
by Jonathan Cameron. Rather than working around the problem
by adding new platform data for this driver, this patch
uses the of_gpio framework to get to the gpio number.

However, the stargate2 code does not (yet) use DT based
probing, so it is still broken, but at least building
allyesconfig works again.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>