[ARM] orion5x: add rtc-m48t86 to orion5x_defconfig

The TS-7800 can have a M48T86 RTC onboard
Signed-off-by: NAlexander Clouter <alex@digriz.org.uk>

arch/arm/configs/orion5x_defconfig

@@ -1177,7 +1177,7 @@ CONFIG_RTC_DRV_S35390A=y
 # CONFIG_RTC_DRV_DS1553 is not set
 # CONFIG_RTC_DRV_DS1742 is not set
 # CONFIG_RTC_DRV_STK17TA8 is not set
-# CONFIG_RTC_DRV_M48T86 is not set
+CONFIG_RTC_DRV_M48T86=y
 # CONFIG_RTC_DRV_M48T59 is not set
 # CONFIG_RTC_DRV_V3020 is not set