atmel: move ATMEL_MAX_UART to platform_data/atmel.h

Modify both AT91 and AVR32 platforms.
Use 7 for it as the sam9260 or the sam9g25 have 7 of them DBGU included.

Reported-by: Joachim Eastwood <joachim.eastwood@jotron.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>