powerpc/defconfig: update RTC support

- remove CONFIG_RTC_DRV_CMOS in corenet32_smp_defconfig(it's unused),
  reserve CONFIG_RTC_DRV_CMOS in mpc85xx_defconfig(needed on some CDS boards)

- enable CONFIG_RTC_DRV_DS1307, CONFIG_RTC_DRV_DS1374,
  CONFIG_RTC_DRV_DS3232 in mpc85xx_defconfig, mpc85xx_smp_defconfig

- enable RTC support in  corenet64_smp_defconfig

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>