powerpc/pseries: enable RTC class support

powerpc/pseries: enable RTC class support

A working rtc kernel driver is needed so that hwclock can synchronize
system clock to rtc during shutdown/boot. We already have a powernv
platform rtc driver located at drivers/rtc/rtc-opal.c. However it depends
on CONFIG_RTC_CLASS which is disabled by default. Hence the driver isn't
enabled and not compiled for the powernv kernel.

We fix this by enabling rtc class support in pseries defconfig which
enables this driver and compiles it into the pseries kernel. In case
CONFIG_PPC_POWERNV is not enabled we fallback to 'Generic RTC support'
driver which emulates the legacy 'PC RTC driver'.

Signed-off-by: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>