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

arch/arm/mach-at91/Kconfig

@@ -154,6 +154,9 @@ config HAVE_AT91_AUDIO_PLL
 config HAVE_AT91_I2S_MUX_CLK
 	bool
 
+config HAVE_AT91_SAM9X60_PLL
+	bool
+
 config SOC_SAM_V4_V5
 	bool