avr32: Clean up the HMATRIX code

Rework the HMATRIX configuration interface so that it becomes easier
to configure the HMATRIX for boards with special needs, and add new
parts.

The HMATRIX header file has been split into a general,
chip-independent part with register definitions, etc. and a
chip-specific part with SFR bitfield definitions and master/slave
identifiers.

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