mpc85xx: p2020: add RTC ds1307 to kernel

If the RTC module is compiled as a module, the hctosys fails to
initialize because ds1307 is loaded later.

Fixes:
[    2.004145] hctosys: unable to open rtc device (rtc0)
[   11.957997] rtc-ds1307 0-006f: registered as rtc0

This is similar to commit 5481ce9,
which was done for imx6 target.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

target/linux/mpc85xx/p2020/config-default

@@ -21,6 +21,7 @@ CONFIG_PCI_MSI=y
 CONFIG_PPC_MSI_BITMAP=y
 CONFIG_RFS_ACCEL=y
 CONFIG_RPS=y
+CONFIG_RTC_DRV_DS1307=y
 CONFIG_RWSEM_SPIN_ON_OWNER=y
 CONFIG_SMP=y
 CONFIG_TREE_RCU=y