ARM: at91: Kconfig: implement PIT64B selection

Implement PIT64B selection thus it will be available for the necessary
targets (at the moment SAM9X60 and SAMA7G5) w/o the necessity to
specify it via defconfig. With this the current CONFIG_TIMER_OF
dependency of PIT64B driver could be removed.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>