avr32: replace simple_strtoul() with kstrtoul()

simple_strtoul() is marked for obsoletion; use the newer and more
pleasant kstrtoul() in its place.

Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Haavard Skinnemoen <hskinnemoen@gmail.com>
Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>