powerpc/83xx: add RTC drivers in 83xx defconfig

83xx boards typically have the DS1307 or DS1374:

git grep RTC_DRV arch/powerpc/configs/*83* | grep "=y"
arch/powerpc/configs/83xx/asp8347_defconfig:CONFIG_RTC_DRV_DS1374=y
arch/powerpc/configs/83xx/mpc8313_rdb_defconfig:CONFIG_RTC_DRV_DS1307=y
arch/powerpc/configs/83xx/mpc8315_rdb_defconfig:CONFIG_RTC_DRV_DS1307=y
arch/powerpc/configs/83xx/mpc832x_mds_defconfig:CONFIG_RTC_DRV_DS1374=y
arch/powerpc/configs/83xx/mpc834x_itx_defconfig:CONFIG_RTC_DRV_DS1307=y
arch/powerpc/configs/83xx/mpc834x_itxgp_defconfig:CONFIG_RTC_DRV_DS1307=y
arch/powerpc/configs/83xx/mpc834x_mds_defconfig:CONFIG_RTC_DRV_DS1374=y
arch/powerpc/configs/83xx/mpc836x_mds_defconfig:CONFIG_RTC_DRV_DS1374=y

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>