kernel: add config option

When the kmod-at91-adc package is activated for the at91 target the new
option CONFIG_AT91_SAMA5D2_ADC is selectable and not handled. Add this
option to the kernel 4.9 configuration.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/generic/config-4.9

@@ -324,6 +324,7 @@ CONFIG_ARM_GIC_MAX_NR=1
 # CONFIG_ASYNC_TX_DMA is not set
 # CONFIG_AT76C50X_USB is not set
 # CONFIG_AT803X_PHY is not set
+# CONFIG_AT91_SAMA5D2_ADC is not set
 # CONFIG_ATA is not set
 # CONFIG_ATAGS is not set
 CONFIG_ATAGS_PROC=y