mfd: aat2870-core: Stop using obsolte simple_strtoul()

Soothes checkpatch warning:

    WARNING: simple_strtoul is obsolete, use kstrtoul instead
    #306: FILE: drivers/mfd/aat2870-core.c:306:
    +	addr = simple_strtoul(start, &start, 16);

Signed-off-by: Lee Jones <lee.jones@linaro.org>