sama5d3_xplained: updated defconfig

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
xplained demo image: force load of g_serial module

For having a console on the usb gadget, we force the load of
atmel_usba_udc and g_serial.

They are declared in /etc/modules file.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
sama5d3_xplained: add both ttyS0 and ttyGS0 as console

Use the SERIAL_CONSOLES variable to specify both serial and
usb gadget serial consoles.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>