USART mapping used to be accomplished by the manual filling of
at32_usart_map[] and at32_nr_usarts.  This has now been replaced
with at32_map_usart() so we can remove these variables.
Signed-off-by: Ben Nizette <ben.nizette@iinet.net.au>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>