AVR32: Rename at32ap7000 -> at32ap700x

The SoC-specific code for all the AT32AP700x CPUs is practically
identical; the only difference is that some chips have less features
than others. By doing this rename, we can add support for the AP7000
derivatives simply by making some features conditional.

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