ARM: at91: Kconfig: add sam9x60 pll config flag

Add SAM9X60's pll config flag. It was first used in
commit a436c2a4 ("clk: at91: add sam9x60 PLL driver").

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1575035505-6310-2-git-send-email-claudiu.beznea@microchip.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>