mfd: ab8500-core: Add Interrupt support for ab8540

ITSource/ITLatch 7, 8, 9 and 10 don't exist on AB8540. This patch
replaces them with '-1' in the interrupt list, and handles the '-1'
in the code accordingly.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>