Add ATSTK100[234] to MAINTAINERS

Add all the ATSTK1000 daughterboards to MAINTAINERS along with their
"mother". Also update the entry for ATSTK1000 to be not only about the
AP7000 CPU; it's intended to handle all CPUs in the AT32AP family.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>