avr32: Move setup_platform() into chip-specific file

Combine at32_clock_init() and at32_portmux_init() into
setup_platform() and remove setup_platform() from at32ap.c. No
functional change since all setup_platform() ever did was call those
two functions.

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