powerpc/configs/c2k: Switch CONFIG_GEN_RTC from =m to =y

In commit 835ea93e ("char/genrtc: remove powerpc support"),
CONFIG_GEN_RTC switch from tristate to bool, update the defconfig to
match.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/configs/c2k_defconfig

@@ -27,6 +27,7 @@ CONFIG_CPU_FREQ_DEFAULT_GOV_USERSPACE=y
 CONFIG_CPU_FREQ_GOV_PERFORMANCE=y
 CONFIG_CPU_FREQ_GOV_POWERSAVE=m
 CONFIG_CPU_FREQ_GOV_ONDEMAND=m
+CONFIG_GEN_RTC=y
 CONFIG_HIGHMEM=y
 CONFIG_PREEMPT_VOLUNTARY=y
 CONFIG_BINFMT_MISC=y
@@ -208,7 +209,6 @@ CONFIG_SERIAL_NONSTANDARD=y
 CONFIG_SERIAL_MPSC=y
 CONFIG_SERIAL_MPSC_CONSOLE=y
 CONFIG_NVRAM=m
-CONFIG_GEN_RTC=m
 CONFIG_RAW_DRIVER=y
 CONFIG_MAX_RAW_DEVS=8192
 CONFIG_I2C=m