pinctrl-bcm2835: Only request the interrupts listed in the DTB

Although the GPIO controller can generate three interrupts (four counting
the common one), the device tree files currently only specify two. In the
absence of the third, simply don't register that interrupt (as opposed to
registering 0), which has the effect of making it impossible to generate
interrupts for GPIOs 46-53 which, since they share pins with the SD card
interface, is unlikely to be a problem.