mfd: ab8500-core: Use devm_* memory/IRQ and allocation/free routines

It is better to use devm_* calls, as they allow for easier
and more automatic clean-up. Resources are device allocated,
so when a device is freed, so are all associated resources.

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