tty/serial: atmel: increase ATMEL_MAX_UART

The samx7 family uses the same UART/USART IP as the at91/sama5 families but
has 8 of those.

Suggested-by: Szemző András <sza@esh.hu>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>