MAINTAINERS: update AVR32 and AT32AP maintainers

This alters the maintenance of the AVR32 architecture and the AT32AP machine
code to be shared between Haavard Skinnemoen and me. The status is also changed
to maintained, as we no longer are being paid to look after this architecture.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Acked-by: Haavard Skinnemoen <hskinnemoen@gmail.com>