configs: sama5d2: enable option CONFIG_ATMEL_TCB_TIMER

Enable CONFIG_ATMEL_TCB_TIMER and disable CONFIG_ATMEL_PIT_TIMER.
This will allow using the TCB timer instead of the PIT one when running
under OP-TEE.

Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Clément Léger <clement.leger@bootlin.com>