avr32/at32ap: fix mapping of platform device id for USART

This patch will fix the mapping of the platform device id when mapping USART
peripheral ID to UART platform device id. Not setting the platform device id
will in most cases (when you map USART > 0 to UART 0) make the console not
available.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>